$ATI
ATI INC Stock $ati

$ati Overview Financials Valuation SEC Filings Company Website

2020 Q4 Form 10-K Financial Statement

#000162828021003434 Filed on February 26, 2021

Income Statement

Concept	2020 Q4	2020	2019 Q4
Revenue	$658.3M	$2.982B	$1.019B
YoY Change	-35.37%	-27.66%	-1.86%
Cost Of Revenue	$613.1M	$2.689B	$849.3M
YoY Change	-27.81%	-22.82%	-4.61%
Gross Profit	$45.20M	$292.8M	$169.3M
YoY Change	-73.3%	-54.11%	14.7%
Gross Profit Margin	6.87%	9.82%	16.62%
Selling, General & Admin	$52.80M	$201.0M	$66.30M
YoY Change	-20.36%	-24.78%	-9.05%
% of Gross Profit	116.81%	68.65%	39.16%
Research & Development		$14.10M
YoY Change		-20.79%
% of Gross Profit		4.82%
Depreciation & Amortization	$35.00M	$143.3M	$36.50M
YoY Change	-4.11%	-5.16%	-7.12%
% of Gross Profit	77.43%	48.94%	21.56%
Operating Expenses	$52.80M	$215.1M	$66.30M
YoY Change	-20.36%	-24.53%	-9.05%
Operating Profit	-$7.600M	-$1.303B	$103.0M
YoY Change	-107.38%	-455.64%	37.88%
Interest Expense	-$26.10M	-$101.4M	-$26.10M
YoY Change	0.0%	-7.57%	3.57%
% of Operating Profit			-25.34%
Other Income/Expense, Net	-$28.20M	-$1.200M	-$18.30M
YoY Change	54.1%	-101.86%	77.67%
Pretax Income	-$1.143B	-$1.482B	$21.20M
YoY Change	-5490.57%	-726.6%	-45.92%
Income Tax	-$26.50M	$77.70M	-$38.80M
% Of Pretax Income			-183.02%
Net Earnings	-$1.121B	-$1.573B	$56.50M
YoY Change	-2084.07%	-722.81%	37.47%
Net Earnings / Revenue	-170.29%	-52.73%	5.55%
Basic Earnings Per Share	-$8.85	-$12.43	$0.45
Diluted Earnings Per Share	-$8.85	-$12.43	$0.41
COMMON SHARES
Basic Shares Outstanding	126.8M shares	126.5M shares	126.1M shares
Diluted Shares Outstanding		126.5M shares

Balance Sheet

Concept	2020 Q4	2020	2019 Q4
SHORT-TERM ASSETS
Cash & Short-Term Investments	$645.9M	$645.9M	$490.8M
YoY Change	31.6%	31.6%	28.48%
Cash & Equivalents	$645.9M		$490.8M
Short-Term Investments		$487.7M
Other Short-Term Assets	$38.30M	$38.30M	$64.30M
YoY Change	-40.44%	-40.44%	-13.81%
Inventory	$997.1M	$997.1M	$1.155B
Prepaid Expenses
Receivables	$345.8M	$384.7M	$554.1M
Other Receivables	$0.00	$0.00	$0.00
Total Short-Term Assets	$2.066B	$2.066B	$2.303B
YoY Change	-10.29%	-10.29%	2.51%
LONG-TERM ASSETS
Property, Plant & Equipment	$1.469B	$1.469B	$2.450B
YoY Change	-40.04%	-41.27%	-1.01%
Goodwill	$240.7M		$525.8M
YoY Change	-54.22%		-1.66%
Intangibles
YoY Change
Long-Term Investments			$0.00
YoY Change
Other Assets	$259.0M	$171.1M	$355.7M
YoY Change	-27.19%	-17.62%	44.95%
Total Long-Term Assets	$1.969B	$1.969B	$3.332B
YoY Change	-40.9%	-40.9%	2.35%
TOTAL ASSETS
Total Short-Term Assets	$2.066B	$2.066B	$2.303B
Total Long-Term Assets	$1.969B	$1.969B	$3.332B
Total Assets	$4.035B	$4.035B	$5.635B
YoY Change	-28.39%	-28.39%	2.41%
SHORT-TERM LIABILITIES
YoY Change
Accounts Payable	$290.6M	$290.6M	$521.2M
YoY Change	-44.24%	-44.24%	4.49%
Accrued Expenses	$233.1M	$110.8M	$237.8M
YoY Change	-1.98%	-16.06%	-8.57%
Deferred Revenue	$111.8M		$78.70M
YoY Change	42.06%		10.22%
Short-Term Debt	$0.00	$0.00	$0.00
YoY Change
Long-Term Debt Due	$17.80M	$17.80M	$11.50M
YoY Change	54.78%	54.78%	74.24%
Total Short-Term Liabilities	$653.3M	$653.3M	$849.2M
YoY Change	-23.07%	-23.07%	1.47%
LONG-TERM LIABILITIES
Long-Term Debt	$1.550B	$1.550B	$1.387B
YoY Change	11.72%	11.72%	-9.65%
Other Long-Term Liabilities	$189.9M	$1.190B	$160.8M
YoY Change	18.1%	-1.21%	79.87%
Total Long-Term Liabilities	$189.9M	$2.740B	$160.8M
YoY Change	18.1%	5.71%	79.87%
TOTAL LIABILITIES
Total Short-Term Liabilities	$653.3M	$653.3M	$849.2M
Total Long-Term Liabilities	$189.9M	$2.740B	$160.8M
Total Liabilities	$3.394B	$3.514B	$3.441B
YoY Change	-1.39%	-0.87%	-1.96%
SHAREHOLDERS EQUITY
Retained Earnings	$106.5M		$1.679B
YoY Change	-93.66%		17.67%
Common Stock	$1.638B		$1.631B
YoY Change	0.46%		0.16%
Preferred Stock
YoY Change
Treasury Stock (at cost)	$0.00		$18.20M
YoY Change	-100.0%		-40.52%
Treasury Stock Shares	2.672K shares		609.8K shares
Shareholders Equity	$521.1M	$521.1M	$2.090B
YoY Change
Total Liabilities & Shareholders Equity	$4.035B	$4.035B	$5.635B
YoY Change	-28.39%	-28.39%	2.41%

Cashflow Statement

Concept	2020 Q4	2020	2019 Q4
OPERATING ACTIVITIES
Net Income	-$1.121B	-$1.573B	$56.50M
YoY Change	-2084.07%	-722.81%	37.47%
Depreciation, Depletion And Amortization	$35.00M	$143.3M	$36.50M
YoY Change	-4.11%	-5.16%	-7.12%
Cash From Operating Activities	$121.9M	$166.9M	$240.1M
YoY Change	-49.23%	-27.47%	-13.07%
INVESTING ACTIVITIES
Capital Expenditures	-$41.90M	-$136.5M	-$70.10M
YoY Change	-40.23%	-18.94%	84.96%
Acquisitions		$0.00
YoY Change
Other Investing Activities	$2.400M	-$1.900M	$2.400M
YoY Change	0.0%	-1050.0%	26.32%
Cash From Investing Activities	-$39.50M	-$128.7M	-$67.70M
YoY Change	-41.65%	-257.53%	88.06%
FINANCING ACTIVITIES
Cash Dividend Paid
YoY Change
Common Stock Issuance & Retirement, Net
YoY Change
Debt Paid & Issued, Net
YoY Change
Cash From Financing Activities	-8.700M	$116.9M	-192.9M
YoY Change	-95.49%	-157.59%	1548.72%
NET CHANGE
Cash From Operating Activities	121.9M	$166.9M	240.1M
Cash From Investing Activities	-39.50M	-$128.7M	-67.70M
Cash From Financing Activities	-8.700M	$116.9M	-192.9M
Net Change In Cash	73.70M	$155.1M	-20.50M
YoY Change	-459.51%	42.56%	-108.97%
FREE CASH FLOW
Cash From Operating Activities	$121.9M	$166.9M	$240.1M
Capital Expenditures	-$41.90M	-$136.5M	-$70.10M
Free Cash Flow	$163.8M	$303.4M	$310.2M
YoY Change	-47.2%	-23.86%	-1.24%

Facts In Submission

Frame	Concept Type	Concept / XBRL Key	Value	Unit
CY2020	dei	Amendment Flag AmendmentFlag	false
CY2020	dei	Entity Central Index Key EntityCentralIndexKey	0001018963
CY2020	dei	Document Fiscal Year Focus DocumentFiscalYearFocus	2020
CY2020	dei	Document Fiscal Period Focus DocumentFiscalPeriodFocus	FY
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CY2020	dei	Document Type DocumentType	10-K
CY2020	dei	Document Annual Report DocumentAnnualReport	true
CY2020	dei	Document Period End Date DocumentPeriodEndDate	2020-12-31
CY2020	dei	Current Fiscal Year End Date CurrentFiscalYearEndDate	--12-31
CY2020	dei	Document Transition Report DocumentTransitionReport	false
CY2020	dei	Entity File Number EntityFileNumber	1-12001
CY2020	dei	Entity Registrant Name EntityRegistrantName	ALLEGHENY TECHNOLOGIES INCORPORATED
CY2020	dei	Entity Incorporation State Country Code EntityIncorporationStateCountryCode	DE
CY2020	dei	Entity Tax Identification Number EntityTaxIdentificationNumber	25-1792394
CY2020	dei	Entity Address Address Line1 EntityAddressAddressLine1	1000 Six PPG Place
CY2020	dei	Entity Address City Or Town EntityAddressCityOrTown	Pittsburgh,
CY2020	dei	Entity Address State Or Province EntityAddressStateOrProvince	PA
CY2020	dei	Entity Address Postal Zip Code EntityAddressPostalZipCode	15222-5479
CY2020	dei	City Area Code CityAreaCode	412
CY2020	dei	Local Phone Number LocalPhoneNumber	394-2800
CY2020	dei	Security12b Title Security12bTitle	Common stock, par value $0.10
CY2020	dei	Trading Symbol TradingSymbol	ATI
CY2020	dei	Security Exchange Name SecurityExchangeName	NYSE
CY2020	dei	Entity Well Known Seasoned Issuer EntityWellKnownSeasonedIssuer	Yes
CY2020	dei	Entity Voluntary Filers EntityVoluntaryFilers	No
CY2020	dei	Entity Current Reporting Status EntityCurrentReportingStatus	Yes
CY2020	dei	Entity Interactive Data Current EntityInteractiveDataCurrent	Yes
CY2020	dei	Entity Filer Category EntityFilerCategory	Large Accelerated Filer
CY2020	dei	Entity Small Business EntitySmallBusiness	false
CY2020	dei	Entity Emerging Growth Company EntityEmergingGrowthCompany	false
CY2020	dei	Icfr Auditor Attestation Flag IcfrAuditorAttestationFlag	true
CY2020	dei	Entity Shell Company EntityShellCompany	false
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CY2020Q4	us-gaap	Assets Current AssetsCurrent	2066000000.0	USD
CY2019Q4	us-gaap	Assets Current AssetsCurrent	2303000000.0	USD
CY2020Q4	us-gaap	Property Plant And Equipment Net PropertyPlantAndEquipmentNet	1469200000	USD
CY2019Q4	us-gaap	Property Plant And Equipment Net PropertyPlantAndEquipmentNet	2450100000	USD
CY2020Q4	us-gaap	Goodwill Goodwill	240700000	USD
CY2019Q4	us-gaap	Goodwill Goodwill	525800000	USD
CY2020Q4	us-gaap	Other Assets Noncurrent OtherAssetsNoncurrent	259000000.0	USD
CY2019Q4	us-gaap	Other Assets Noncurrent OtherAssetsNoncurrent	355700000	USD
CY2020Q4	us-gaap	Assets Assets	4034900000	USD
CY2019Q4	us-gaap	Assets Assets	5634600000	USD
CY2020Q4	us-gaap	Accounts Payable Current AccountsPayableCurrent	290600000	USD
CY2019Q4	us-gaap	Accounts Payable Current AccountsPayableCurrent	521200000	USD
CY2020Q4	us-gaap	Contract With Customer Liability Current ContractWithCustomerLiabilityCurrent	111800000	USD
CY2019Q4	us-gaap	Contract With Customer Liability Current ContractWithCustomerLiabilityCurrent	78700000	USD
CY2020Q4	us-gaap	Debt Current DebtCurrent	17800000	USD
CY2019Q4	us-gaap	Debt Current DebtCurrent	11500000	USD
CY2020Q4	us-gaap	Other Liabilities Current OtherLiabilitiesCurrent	233100000	USD
CY2019Q4	us-gaap	Other Liabilities Current OtherLiabilitiesCurrent	237800000	USD
CY2020Q4	us-gaap	Liabilities Current LiabilitiesCurrent	653300000	USD
CY2019Q4	us-gaap	Liabilities Current LiabilitiesCurrent	849200000	USD
CY2020Q4	us-gaap	Long Term Debt And Capital Lease Obligations LongTermDebtAndCapitalLeaseObligations	1550000000.0	USD
CY2019Q4	us-gaap	Long Term Debt And Capital Lease Obligations LongTermDebtAndCapitalLeaseObligations	1387400000	USD
CY2020Q4	us-gaap	Other Postretirement Defined Benefit Plan Liabilities Noncurrent OtherPostretirementDefinedBenefitPlanLiabilitiesNoncurrent	326700000	USD
CY2019Q4	us-gaap	Other Postretirement Defined Benefit Plan Liabilities Noncurrent OtherPostretirementDefinedBenefitPlanLiabilitiesNoncurrent	312500000	USD
CY2019	us-gaap	Depreciation Depletion And Amortization DepreciationDepletionAndAmortization	151100000	USD
CY2020Q4	us-gaap	Defined Benefit Pension Plan Liabilities Noncurrent DefinedBenefitPensionPlanLiabilitiesNoncurrent	673600000	USD
CY2019Q4	us-gaap	Defined Benefit Pension Plan Liabilities Noncurrent DefinedBenefitPensionPlanLiabilitiesNoncurrent	731500000	USD
CY2020Q4	us-gaap	Other Liabilities Noncurrent OtherLiabilitiesNoncurrent	189900000	USD
CY2019Q4	us-gaap	Other Liabilities Noncurrent OtherLiabilitiesNoncurrent	160800000	USD
CY2020Q4	us-gaap	Liabilities Liabilities	3393500000	USD
CY2019Q4	us-gaap	Liabilities Liabilities	3441400000	USD
CY2020Q4	us-gaap	Preferred Stock Par Or Stated Value Per Share PreferredStockParOrStatedValuePerShare	0.10
CY2019Q4	us-gaap	Preferred Stock Par Or Stated Value Per Share PreferredStockParOrStatedValuePerShare	0.10
CY2020Q4	us-gaap	Preferred Stock Shares Authorized PreferredStockSharesAuthorized	50000000	shares
CY2019Q4	us-gaap	Preferred Stock Shares Authorized PreferredStockSharesAuthorized	50000000	shares
CY2020Q4	us-gaap	Preferred Stock Shares Issued PreferredStockSharesIssued	0	shares
CY2019Q4	us-gaap	Preferred Stock Shares Issued PreferredStockSharesIssued	0	shares
CY2020Q4	us-gaap	Preferred Stock Value PreferredStockValue	0	USD
CY2019Q4	us-gaap	Preferred Stock Value PreferredStockValue	0	USD
CY2019Q4	us-gaap	Common Stock Par Or Stated Value Per Share CommonStockParOrStatedValuePerShare	0.10
CY2020Q4	us-gaap	Common Stock Par Or Stated Value Per Share CommonStockParOrStatedValuePerShare	0.10
CY2020Q4	us-gaap	Common Stock Shares Authorized CommonStockSharesAuthorized	500000000	shares
CY2019Q4	us-gaap	Common Stock Shares Authorized CommonStockSharesAuthorized	500000000	shares
CY2020Q4	us-gaap	Common Stock Shares Issued CommonStockSharesIssued	126820440	shares
CY2019Q4	us-gaap	Common Stock Shares Issued CommonStockSharesIssued	126695171	shares
CY2020Q4	us-gaap	Common Stock Shares Outstanding CommonStockSharesOutstanding	126817768	shares
CY2019Q4	us-gaap	Common Stock Shares Outstanding CommonStockSharesOutstanding	126085348	shares
CY2020Q4	us-gaap	Common Stock Value CommonStockValue	12700000	USD
CY2019Q4	us-gaap	Common Stock Value CommonStockValue	12700000	USD
CY2020Q4	us-gaap	Additional Paid In Capital Common Stock AdditionalPaidInCapitalCommonStock	1625500000	USD
CY2019Q4	us-gaap	Additional Paid In Capital Common Stock AdditionalPaidInCapitalCommonStock	1618000000.0	USD
CY2020Q4	us-gaap	Retained Earnings Accumulated Deficit RetainedEarningsAccumulatedDeficit	106500000	USD
CY2019Q4	us-gaap	Retained Earnings Accumulated Deficit RetainedEarningsAccumulatedDeficit	1679300000	USD
CY2020Q4	us-gaap	Treasury Stock Shares TreasuryStockShares	2672	shares
CY2019Q4	us-gaap	Treasury Stock Shares TreasuryStockShares	609823	shares
CY2020Q4	us-gaap	Treasury Stock Value TreasuryStockValue	0	USD
CY2019Q4	us-gaap	Treasury Stock Value TreasuryStockValue	18200000	USD
CY2020Q4	us-gaap	Accumulated Other Comprehensive Income Loss Net Of Tax AccumulatedOtherComprehensiveIncomeLossNetOfTax	-1223600000	USD
CY2019Q4	us-gaap	Accumulated Other Comprehensive Income Loss Net Of Tax AccumulatedOtherComprehensiveIncomeLossNetOfTax	-1201700000	USD
CY2020Q4	us-gaap	Stockholders Equity StockholdersEquity	521100000	USD
CY2019Q4	us-gaap	Stockholders Equity StockholdersEquity	2090100000	USD
CY2020Q4	us-gaap	Minority Interest MinorityInterest	120300000	USD
CY2019Q4	us-gaap	Minority Interest MinorityInterest	103100000	USD
CY2020Q4	us-gaap	Stockholders Equity Including Portion Attributable To Noncontrolling Interest StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest	641400000	USD
CY2019Q4	us-gaap	Stockholders Equity Including Portion Attributable To Noncontrolling Interest StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest	2193200000	USD
CY2020Q4	us-gaap	Liabilities And Stockholders Equity LiabilitiesAndStockholdersEquity	4034900000	USD
CY2019Q4	us-gaap	Liabilities And Stockholders Equity LiabilitiesAndStockholdersEquity	5634600000	USD
CY2020	us-gaap	Profit Loss ProfitLoss	-1559600000	USD
CY2019	us-gaap	Profit Loss ProfitLoss	270100000	USD
CY2018	us-gaap	Profit Loss ProfitLoss	236700000	USD
CY2020	us-gaap	Depreciation Depletion And Amortization DepreciationDepletionAndAmortization	143300000	USD
CY2018	us-gaap	Depreciation Depletion And Amortization DepreciationDepletionAndAmortization	156400000	USD
CY2020	us-gaap	Deferred Income Taxes And Tax Credits DeferredIncomeTaxesAndTaxCredits	69900000	USD
CY2019	us-gaap	Deferred Income Taxes And Tax Credits DeferredIncomeTaxesAndTaxCredits	-40900000	USD
CY2018	us-gaap	Deferred Income Taxes And Tax Credits DeferredIncomeTaxesAndTaxCredits	2100000	USD
CY2020	us-gaap	Deconsolidation Gain Or Loss Amount DeconsolidationGainOrLossAmount	0	USD
CY2019	us-gaap	Deconsolidation Gain Or Loss Amount DeconsolidationGainOrLossAmount	0	USD
CY2018	us-gaap	Deconsolidation Gain Or Loss Amount DeconsolidationGainOrLossAmount	15900000	USD
CY2020	us-gaap	Goodwill Impairment Loss GoodwillImpairmentLoss	287000000.0	USD
CY2019	us-gaap	Goodwill Impairment Loss GoodwillImpairmentLoss	0	USD
CY2018	us-gaap	Goodwill Impairment Loss GoodwillImpairmentLoss	0	USD
CY2020	us-gaap	Gains Losses On Extinguishment Of Debt GainsLossesOnExtinguishmentOfDebt	-21500000	USD
CY2019	us-gaap	Gains Losses On Extinguishment Of Debt GainsLossesOnExtinguishmentOfDebt	-21600000	USD
CY2018	us-gaap	Gains Losses On Extinguishment Of Debt GainsLossesOnExtinguishmentOfDebt	0	USD
CY2020	us-gaap	Gain Loss On Sale Of Property Plant Equipment GainLossOnSaleOfPropertyPlantEquipment	2900000	USD
CY2019	us-gaap	Gain Loss On Sale Of Property Plant Equipment GainLossOnSaleOfPropertyPlantEquipment	90600000	USD
CY2018	us-gaap	Gain Loss On Sale Of Property Plant Equipment GainLossOnSaleOfPropertyPlantEquipment	1200000	USD
CY2020	us-gaap	Disposal Group Not Discontinued Operation Gain Loss On Disposal DisposalGroupNotDiscontinuedOperationGainLossOnDisposal	0	USD
CY2019	us-gaap	Disposal Group Not Discontinued Operation Gain Loss On Disposal DisposalGroupNotDiscontinuedOperationGainLossOnDisposal	-1800000	USD
CY2018	us-gaap	Disposal Group Not Discontinued Operation Gain Loss On Disposal DisposalGroupNotDiscontinuedOperationGainLossOnDisposal	0	USD
CY2020	us-gaap	Other Asset Impairment Charges OtherAssetImpairmentCharges	1041500000	USD
CY2019	us-gaap	Other Asset Impairment Charges OtherAssetImpairmentCharges	0	USD
CY2018	us-gaap	Other Asset Impairment Charges OtherAssetImpairmentCharges	0	USD
CY2020	ati	Non Cash Joint Venture Impairment Charge NonCashJointVentureImpairmentCharge	0	USD
CY2019	ati	Non Cash Joint Venture Impairment Charge NonCashJointVentureImpairmentCharge	11400000	USD
CY2018	ati	Non Cash Joint Venture Impairment Charge NonCashJointVentureImpairmentCharge	0	USD
CY2020	us-gaap	Increase Decrease In Pension And Postretirement Obligations IncreaseDecreaseInPensionAndPostretirementObligations	-91900000	USD
CY2019	us-gaap	Increase Decrease In Pension And Postretirement Obligations IncreaseDecreaseInPensionAndPostretirementObligations	-103300000	USD
CY2018	us-gaap	Increase Decrease In Pension And Postretirement Obligations IncreaseDecreaseInPensionAndPostretirementObligations	-32600000	USD
CY2020	us-gaap	Increase Decrease In Accounts Receivable IncreaseDecreaseInAccountsReceivable	-208400000	USD
CY2019	us-gaap	Increase Decrease In Accounts Receivable IncreaseDecreaseInAccountsReceivable	52100000	USD
CY2018	us-gaap	Increase Decrease In Accounts Receivable IncreaseDecreaseInAccountsReceivable	-16000000.0	USD
CY2020	us-gaap	Increase Decrease In Inventories IncreaseDecreaseInInventories	-158200000	USD
CY2019	us-gaap	Increase Decrease In Inventories IncreaseDecreaseInInventories	-25400000	USD
CY2020	us-gaap	Increase Decrease In Accounts Payable Trade IncreaseDecreaseInAccountsPayableTrade	-230500000	USD
CY2019	us-gaap	Increase Decrease In Accounts Payable Trade IncreaseDecreaseInAccountsPayableTrade	30100000	USD
CY2018	us-gaap	Increase Decrease In Inventories IncreaseDecreaseInInventories	108500000	USD
CY2018	us-gaap	Increase Decrease In Accounts Payable Trade IncreaseDecreaseInAccountsPayableTrade	153700000	USD
CY2020	us-gaap	Increase Decrease In Accrued Income Taxes Payable IncreaseDecreaseInAccruedIncomeTaxesPayable	2400000	USD
CY2019	us-gaap	Increase Decrease In Accrued Income Taxes Payable IncreaseDecreaseInAccruedIncomeTaxesPayable	4900000	USD
CY2018	us-gaap	Increase Decrease In Accrued Income Taxes Payable IncreaseDecreaseInAccruedIncomeTaxesPayable	1400000	USD
CY2020	us-gaap	Increase Decrease In Other Operating Capital Net IncreaseDecreaseInOtherOperatingCapitalNet	-119600000	USD
CY2019	us-gaap	Increase Decrease In Other Operating Capital Net IncreaseDecreaseInOtherOperatingCapitalNet	-600000	USD
CY2018	us-gaap	Increase Decrease In Other Operating Capital Net IncreaseDecreaseInOtherOperatingCapitalNet	15300000	USD
CY2020	us-gaap	Net Cash Provided By Used In Operating Activities NetCashProvidedByUsedInOperatingActivities	166900000	USD
CY2019	us-gaap	Net Cash Provided By Used In Operating Activities NetCashProvidedByUsedInOperatingActivities	230100000	USD
CY2018	us-gaap	Net Cash Provided By Used In Operating Activities NetCashProvidedByUsedInOperatingActivities	392800000	USD
CY2020	us-gaap	Payments To Acquire Productive Assets PaymentsToAcquireProductiveAssets	136500000	USD
CY2019	us-gaap	Payments To Acquire Productive Assets PaymentsToAcquireProductiveAssets	168200000	USD
CY2018	us-gaap	Payments To Acquire Productive Assets PaymentsToAcquireProductiveAssets	139200000	USD
CY2020	us-gaap	Proceeds From Sale Of Property Plant And Equipment ProceedsFromSaleOfPropertyPlantAndEquipment	5900000	USD
CY2019	us-gaap	Proceeds From Sale Of Property Plant And Equipment ProceedsFromSaleOfPropertyPlantAndEquipment	92000000.0	USD
CY2018	us-gaap	Proceeds From Sale Of Property Plant And Equipment ProceedsFromSaleOfPropertyPlantAndEquipment	2800000	USD
CY2020	us-gaap	Payments To Acquire Businesses Net Of Cash Acquired PaymentsToAcquireBusinessesNetOfCashAcquired	0	USD
CY2019	us-gaap	Payments To Acquire Businesses Net Of Cash Acquired PaymentsToAcquireBusinessesNetOfCashAcquired	0	USD
CY2018	us-gaap	Payments To Acquire Businesses Net Of Cash Acquired PaymentsToAcquireBusinessesNetOfCashAcquired	10000000.0	USD
CY2020	us-gaap	Proceeds From Divestiture Of Businesses And Interests In Affiliates ProceedsFromDivestitureOfBusinessesAndInterestsInAffiliates	0	USD
CY2019	us-gaap	Proceeds From Divestiture Of Businesses And Interests In Affiliates ProceedsFromDivestitureOfBusinessesAndInterestsInAffiliates	158100000	USD
CY2018	us-gaap	Proceeds From Divestiture Of Businesses And Interests In Affiliates ProceedsFromDivestitureOfBusinessesAndInterestsInAffiliates	0	USD
CY2020	us-gaap	Payments For Proceeds From Other Investing Activities PaymentsForProceedsFromOtherInvestingActivities	-1900000	USD
CY2019	us-gaap	Payments For Proceeds From Other Investing Activities PaymentsForProceedsFromOtherInvestingActivities	200000	USD
CY2018	us-gaap	Payments For Proceeds From Other Investing Activities PaymentsForProceedsFromOtherInvestingActivities	-1300000	USD
CY2020	us-gaap	Net Cash Provided By Used In Investing Activities NetCashProvidedByUsedInInvestingActivities	-128700000	USD
CY2019	us-gaap	Net Cash Provided By Used In Investing Activities NetCashProvidedByUsedInInvestingActivities	81700000	USD
CY2018	us-gaap	Net Cash Provided By Used In Investing Activities NetCashProvidedByUsedInInvestingActivities	-145100000	USD
CY2020	us-gaap	Proceeds From Issuance Of Long Term Debt ProceedsFromIssuanceOfLongTermDebt	391400000	USD
CY2019	us-gaap	Proceeds From Issuance Of Long Term Debt ProceedsFromIssuanceOfLongTermDebt	350000000.0	USD
CY2018	us-gaap	Proceeds From Issuance Of Long Term Debt ProceedsFromIssuanceOfLongTermDebt	7100000	USD
CY2020	us-gaap	Repayments Of Long Term Debt And Capital Securities RepaymentsOfLongTermDebtAndCapitalSecurities	212100000	USD
CY2019	us-gaap	Repayments Of Long Term Debt And Capital Securities RepaymentsOfLongTermDebtAndCapitalSecurities	507600000	USD
CY2018	us-gaap	Repayments Of Long Term Debt And Capital Securities RepaymentsOfLongTermDebtAndCapitalSecurities	6400000	USD
CY2020	us-gaap	Proceeds From Repayments Of Other Debt ProceedsFromRepaymentsOfOtherDebt	200000	USD
CY2019	us-gaap	Proceeds From Repayments Of Other Debt ProceedsFromRepaymentsOfOtherDebt	4900000	USD
CY2018	us-gaap	Proceeds From Repayments Of Other Debt ProceedsFromRepaymentsOfOtherDebt	-5900000	USD
CY2020	ati	Payment Of Capped Call Transactions Cost PaymentOfCappedCallTransactionsCost	19400000	USD
CY2019	ati	Payment Of Capped Call Transactions Cost PaymentOfCappedCallTransactionsCost	0	USD
CY2018	ati	Payment Of Capped Call Transactions Cost PaymentOfCappedCallTransactionsCost	0	USD
CY2020	us-gaap	Payments Of Debt Issuance Costs PaymentsOfDebtIssuanceCosts	9100000	USD
CY2019	us-gaap	Payments Of Debt Issuance Costs PaymentsOfDebtIssuanceCosts	5500000	USD
CY2018	us-gaap	Payments Of Debt Issuance Costs PaymentsOfDebtIssuanceCosts	0	USD
CY2020	us-gaap	Payments Of Debt Extinguishment Costs PaymentsOfDebtExtinguishmentCosts	19100000	USD
CY2019	us-gaap	Payments Of Debt Extinguishment Costs PaymentsOfDebtExtinguishmentCosts	20900000	USD
CY2018	us-gaap	Payments Of Debt Extinguishment Costs PaymentsOfDebtExtinguishmentCosts	0	USD
CY2019	us-gaap	Proceeds From Minority Shareholders ProceedsFromMinorityShareholders	0	USD
CY2020	us-gaap	Payments Of Dividends Minority Interest PaymentsOfDividendsMinorityInterest	7200000	USD
CY2019	us-gaap	Payments Of Dividends Minority Interest PaymentsOfDividendsMinorityInterest	14000000.0	USD
CY2018	us-gaap	Payments Of Dividends Minority Interest PaymentsOfDividendsMinorityInterest	10000000.0	USD
CY2020	us-gaap	Proceeds From Minority Shareholders ProceedsFromMinorityShareholders	0	USD
CY2018	us-gaap	Proceeds From Minority Shareholders ProceedsFromMinorityShareholders	14400000	USD
CY2020	us-gaap	Payments For Repurchase Of Other Equity PaymentsForRepurchaseOfOtherEquity	7800000	USD
CY2019	us-gaap	Payments For Repurchase Of Other Equity PaymentsForRepurchaseOfOtherEquity	9900000	USD
CY2018	us-gaap	Payments For Repurchase Of Other Equity PaymentsForRepurchaseOfOtherEquity	6500000	USD
CY2020	us-gaap	Net Cash Provided By Used In Financing Activities NetCashProvidedByUsedInFinancingActivities	116900000	USD
CY2019	us-gaap	Net Cash Provided By Used In Financing Activities NetCashProvidedByUsedInFinancingActivities	-203000000.0	USD
CY2018	us-gaap	Net Cash Provided By Used In Financing Activities NetCashProvidedByUsedInFinancingActivities	-7300000	USD
CY2020	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Period Increase Decrease Including Exchange Rate Effect CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect	155100000	USD
CY2019	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Period Increase Decrease Including Exchange Rate Effect CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect	108800000	USD
CY2018	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Period Increase Decrease Including Exchange Rate Effect CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect	240400000	USD
CY2019Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	490800000	USD
CY2018Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	382000000.0	USD
CY2017Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	141600000	USD
CY2020Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	645900000	USD
CY2019Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	490800000	USD
CY2018Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	382000000.0	USD
CY2017Q4	us-gaap	Stockholders Equity Including Portion Attributable To Noncontrolling Interest StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest	1844500000	USD
CY2018	us-gaap	Profit Loss ProfitLoss	236700000	USD
CY2018	us-gaap	Other Comprehensive Income Loss Net Of Tax OtherComprehensiveIncomeLossNetOfTax	-112200000	USD
CY2018	us-gaap	Minority Interest Decrease From Distributions To Noncontrolling Interest Holders MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders	10000000.0	USD
CY2018	us-gaap	Noncontrolling Interest Increase From Sale Of Parent Equity Interest NoncontrollingInterestIncreaseFromSaleOfParentEquityInterest	2700000	USD
CY2018	ati	Employee Stock Plans EmployeeStockPlans	-14400000	USD
CY2018Q4	us-gaap	Stockholders Equity Including Portion Attributable To Noncontrolling Interest StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest	1991600000	USD
CY2019	us-gaap	Profit Loss ProfitLoss	270100000	USD
CY2019	us-gaap	Other Comprehensive Income Loss Net Of Tax OtherComprehensiveIncomeLossNetOfTax	-69200000	USD
CY2019	us-gaap	Minority Interest Decrease From Distributions To Noncontrolling Interest Holders MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders	14000000.0	USD
CY2019	ati	Employee Stock Plans EmployeeStockPlans	-14700000	USD
CY2019Q4	us-gaap	Stockholders Equity Including Portion Attributable To Noncontrolling Interest StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest	2193200000	USD
CY2020	us-gaap	Profit Loss ProfitLoss	-1559600000	USD
CY2020	us-gaap	Other Comprehensive Income Loss Net Of Tax OtherComprehensiveIncomeLossNetOfTax	-10500000	USD
CY2020	us-gaap	Adjustments To Additional Paid In Capital Equity Component Of Convertible Debt AdjustmentsToAdditionalPaidInCapitalEquityComponentOfConvertibleDebt	49800000	USD
CY2020	ati	Adjustments To Additional Paid In Capital Convertible Note Capped Call AdjustmentsToAdditionalPaidInCapitalConvertibleNoteCappedCall	19400000	USD
CY2020	us-gaap	Minority Interest Decrease From Distributions To Noncontrolling Interest Holders MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders	7200000	USD
CY2020	ati	Employee Stock Plans EmployeeStockPlans	4900000	USD
CY2020Q4	us-gaap	Stockholders Equity Including Portion Attributable To Noncontrolling Interest StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest	641400000	USD
CY2020	us-gaap	Use Of Estimates UseOfEstimates	<div style="margin-top:9pt"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-style:italic;font-weight:700;line-height:120%">Risks and Uncertainties and Use of Estimates</span></div><div style="margin-top:9pt"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:400;line-height:120%">The preparation of consolidated financial statements in conformity with United States generally accepted accounting principles requires management to make estimates and assumptions that affect reported amounts of assets and liabilities at the date of the financial statements, as well as the reported amounts of income and expenses during the reporting period. Actual results could differ from those estimates. Management believes that the estimates are reasonable. Certain prior year amounts have been reclassified in order to conform with the 2020 presentation.</span></div>
CY2020	us-gaap	Concentration Risk Credit Risk ConcentrationRiskCreditRisk	The Company markets its products to a diverse customer base, principally throughout the United States. No single customer accounted for more than 10% of sales for any year presented. The major end markets for the ATI’s products are customers in the aerospace & defense, energy, automotive, construction and mining, food equipment and appliances, and medical markets.
CY2020Q4	dei	Entity Number Of Employees EntityNumberOfEmployees	6500	employee
CY2020	ati	Employees Percent Non U.S. EmployeesPercentNonU.S.	0.20
CY2020	ati	Employees Percent Coveredby Collective Bargaining Agreements EmployeesPercentCoveredbyCollectiveBargainingAgreements	0.40
CY2020Q4	ati	Entity Number Of Employees Covered By Usw Collective Bargaining Agreements Expiring February282021 EntityNumberOfEmployeesCoveredByUSWCollectiveBargainingAgreementsExpiringFebruary282021	1100	employee
CY2020Q4	us-gaap	Allowance For Doubtful Accounts Receivable AllowanceForDoubtfulAccountsReceivable	4300000	USD
CY2019Q4	us-gaap	Allowance For Doubtful Accounts Receivable AllowanceForDoubtfulAccountsReceivable	4600000	USD
CY2020	ati	Corporate Expenses CorporateExpenses	40900000	USD
CY2020	us-gaap	Research And Development Expense ResearchAndDevelopmentExpense	14100000	USD
CY2019	us-gaap	Research And Development Expense ResearchAndDevelopmentExpense	17800000	USD
CY2018	us-gaap	Research And Development Expense ResearchAndDevelopmentExpense	22700000	USD
CY2020	us-gaap	Research And Development Arrangement Contract To Perform For Others Costs Incurred Gross ResearchAndDevelopmentArrangementContractToPerformForOthersCostsIncurredGross	700000	USD
CY2019	us-gaap	Research And Development Arrangement Contract To Perform For Others Costs Incurred Gross ResearchAndDevelopmentArrangementContractToPerformForOthersCostsIncurredGross	2400000	USD
CY2018	us-gaap	Research And Development Arrangement Contract To Perform For Others Costs Incurred Gross ResearchAndDevelopmentArrangementContractToPerformForOthersCostsIncurredGross	2200000	USD
CY2020	us-gaap	Description Of New Accounting Pronouncements Not Yet Adopted DescriptionOfNewAccountingPronouncementsNotYetAdopted	<div style="margin-top:9pt"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:700;line-height:120%">Pending Accounting Pronouncements</span></div><div style="margin-top:9pt"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:400;line-height:120%">In August 2020, the FASB issued new accounting guidance related to accounting for convertible instruments. Under this new guidance, embedded conversion features are no longer separated from the host contract for convertible instruments with conversion features that are not required to be accounted for as derivatives, or that do not result in substantial premiums accounted for as paid-in capital. As such, a convertible debt instrument will be accounted for as a single liability measured at its amortized cost, as long as no other features require bifurcation and recognition as derivatives. By removing those separation models, the reported interest rate of convertible debt instruments typically will be closer to the coupon interest rate. The new guidance also addresses how convertible instruments are accounted for in the diluted earnings per share calculation, requiring the if-converted method, and requires enhanced disclosures about the terms of convertible instruments and contracts in an entity’s own equity. This new guidance is effective for the Company in fiscal year 2022, with early adoption permitted.</span></div><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:400;line-height:120%">The Company adopted this new accounting guidance related to accounting for convertible instruments effective January 1, 2021 using the modified transition approach with the cumulative effect recognized as an adjustment to the opening balance of retained earnings. This new guidance is applicable to the Company’s 3.5% Convertible Senior Notes due 2025 (the 2025 Convertible Notes) that were issued in June 2020, for which the embedded conversion option was required to be separately accounted for as a component of stockholders’ equity. Upon adoption on January 1, 2021, long-term debt increased by $45.4 million and stockholders’ equity decreased by the same amount, representing the net impact of two adjustments: (1) the $49.8 million value of the embedded conversion, which is net of allocated offering costs, previously classified in additional paid-in-capital in stockholders’ equity, and (2) a $4.4 million increase to retained earnings for the cumulative effect of adoption primarily related to the non-cash interest expense recorded in fiscal year 2020 for the amortization of the portion of the 2025 Convertible Notes allocated to stockholders’ equity. Prospectively, the reported interest expense for the 2025 Convertible Notes will no longer include the non-cash interest expense of the equity component as required under prior accounting standards and will be closer to the 3.5% cash coupon rate.</span><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:12pt;font-weight:400;line-height:120%"> </span><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:400;line-height:120%">There will be no impact to the Company’s earnings per share calculation as it previously applied the if-converted method to the 2025 Convertible Notes given ATI’s flexibility to settle conversions of the 2025 Convertible Notes in cash, shares of ATI’s common stock or a combination thereof, at ATI’s election.</span>
CY2020	ati	Revenue From International Sales RevenueFromInternationalSales	1173000000.0	USD
CY2019	ati	Revenue From International Sales RevenueFromInternationalSales	1667900000	USD
CY2018	ati	Revenue From International Sales RevenueFromInternationalSales	1698400000	USD
CY2020	ati	Entity Wide Disclosure On Geographic Areas Revenue From External Foreign Customers Attributed To Entitys Country Of Domicile EntityWideDisclosureOnGeographicAreasRevenueFromExternalForeignCustomersAttributedToEntitysCountryOfDomicile	812300000	USD
CY2019	ati	Entity Wide Disclosure On Geographic Areas Revenue From External Foreign Customers Attributed To Entitys Country Of Domicile EntityWideDisclosureOnGeographicAreasRevenueFromExternalForeignCustomersAttributedToEntitysCountryOfDomicile	1262600000	USD
CY2018	ati	Entity Wide Disclosure On Geographic Areas Revenue From External Foreign Customers Attributed To Entitys Country Of Domicile EntityWideDisclosureOnGeographicAreasRevenueFromExternalForeignCustomersAttributedToEntitysCountryOfDomicile	1303800000	USD
CY2020	ati	Segment Earnings Before Income Taxes Depreciation Amortization Ebitda SegmentEarningsBeforeIncomeTaxesDepreciationAmortizationEBITDA	244600000	USD
CY2019	ati	Segment Earnings Before Income Taxes Depreciation Amortization Ebitda SegmentEarningsBeforeIncomeTaxesDepreciationAmortizationEBITDA	528800000	USD
CY2018	ati	Segment Earnings Before Income Taxes Depreciation Amortization Ebitda SegmentEarningsBeforeIncomeTaxesDepreciationAmortizationEBITDA	566700000	USD
CY2020	ati	Lifo Provision Benefitand Changein Debit Lifonr Vreservenet LifoProvisionBenefitandChangeinDebitLIFONRVreservenet	0	USD
CY2019	ati	Lifo Provision Benefitand Changein Debit Lifonr Vreservenet LifoProvisionBenefitandChangeinDebitLIFONRVreservenet	100000	USD
CY2018	ati	Lifo Provision Benefitand Changein Debit Lifonr Vreservenet LifoProvisionBenefitandChangeinDebitLIFONRVreservenet	700000	USD
CY2019	ati	Corporate Expenses CorporateExpenses	65300000	USD
CY2018	ati	Corporate Expenses CorporateExpenses	57300000	USD
CY2020	ati	Other Expense OtherExpense	7400000	USD
CY2019	ati	Other Expense OtherExpense	24000000.0	USD
CY2018	ati	Other Expense OtherExpense	19500000	USD
CY2020	ati	Adjusted Earnings Before Income Taxes Depreciation Amortization Ebitda AdjustedEarningsBeforeIncomeTaxesDepreciationAmortizationEBITDA	196300000	USD
CY2019	ati	Adjusted Earnings Before Income Taxes Depreciation Amortization Ebitda AdjustedEarningsBeforeIncomeTaxesDepreciationAmortizationEBITDA	439400000	USD
CY2018	ati	Adjusted Earnings Before Income Taxes Depreciation Amortization Ebitda AdjustedEarningsBeforeIncomeTaxesDepreciationAmortizationEBITDA	489200000	USD
CY2020	us-gaap	Depreciation Depletion And Amortization DepreciationDepletionAndAmortization	143300000	USD
CY2019	us-gaap	Depreciation Depletion And Amortization DepreciationDepletionAndAmortization	151100000	USD
CY2018	us-gaap	Depreciation Depletion And Amortization DepreciationDepletionAndAmortization	156400000	USD
CY2020	ati	Interest Expense Net InterestExpenseNet	94400000	USD
CY2019	ati	Interest Expense Net InterestExpenseNet	99000000.0	USD
CY2018	ati	Interest Expense Net InterestExpenseNet	101000000.0	USD
CY2020	ati	Restructuringandothercharges Restructuringandothercharges	1132100000	USD
CY2019	ati	Restructuringandothercharges Restructuringandothercharges	4500000	USD
CY2018	ati	Restructuringandothercharges Restructuringandothercharges	0	USD
CY2020	us-gaap	Goodwill Impairment Loss GoodwillImpairmentLoss	287000000.0	USD
CY2019	us-gaap	Goodwill Impairment Loss GoodwillImpairmentLoss	0	USD
CY2018	us-gaap	Goodwill Impairment Loss GoodwillImpairmentLoss	0	USD
CY2020	ati	Other Asset Impairment Charges And Due From Affiliates Allowance For Credit Loss OtherAssetImpairmentChargesAndDueFromAffiliatesAllowanceForCreditLoss	2400000	USD
CY2019	ati	Other Asset Impairment Charges And Due From Affiliates Allowance For Credit Loss OtherAssetImpairmentChargesAndDueFromAffiliatesAllowanceForCreditLoss	11400000	USD
CY2018	ati	Other Asset Impairment Charges And Due From Affiliates Allowance For Credit Loss OtherAssetImpairmentChargesAndDueFromAffiliatesAllowanceForCreditLoss	0	USD
CY2020	us-gaap	Deconsolidation Gain Or Loss Amount DeconsolidationGainOrLossAmount	0	USD
CY2019	us-gaap	Deconsolidation Gain Or Loss Amount DeconsolidationGainOrLossAmount	0	USD
CY2018	us-gaap	Deconsolidation Gain Or Loss Amount DeconsolidationGainOrLossAmount	15900000	USD
CY2020	us-gaap	Gains Losses On Extinguishment Of Debt GainsLossesOnExtinguishmentOfDebt	-21500000	USD
CY2019	us-gaap	Gains Losses On Extinguishment Of Debt GainsLossesOnExtinguishmentOfDebt	-21600000	USD
CY2018	us-gaap	Gains Losses On Extinguishment Of Debt GainsLossesOnExtinguishmentOfDebt	0	USD
CY2020	us-gaap	Gain Loss On Sales Of Assets And Asset Impairment Charges GainLossOnSalesOfAssetsAndAssetImpairmentCharges	2500000	USD
CY2019	us-gaap	Gain Loss On Sales Of Assets And Asset Impairment Charges GainLossOnSalesOfAssetsAndAssetImpairmentCharges	89800000	USD
CY2018	us-gaap	Gain Loss On Sales Of Assets And Asset Impairment Charges GainLossOnSalesOfAssetsAndAssetImpairmentCharges	0	USD
CY2020	us-gaap	Income Loss From Continuing Operations Before Income Taxes Extraordinary Items Noncontrolling Interest IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	-1481900000	USD
CY2019	us-gaap	Income Loss From Continuing Operations Before Income Taxes Extraordinary Items Noncontrolling Interest IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	241600000	USD
CY2019	us-gaap	Gain Loss On Sales Of Assets And Asset Impairment Charges GainLossOnSalesOfAssetsAndAssetImpairmentCharges	89800000	USD
CY2018	us-gaap	Income Loss From Continuing Operations Before Income Taxes Extraordinary Items Noncontrolling Interest IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	247700000	USD
CY2020	us-gaap	Gain Loss On Sales Of Assets And Asset Impairment Charges GainLossOnSalesOfAssetsAndAssetImpairmentCharges	2500000	USD
CY2019	us-gaap	Gain Loss On Sale Of Property GainLossOnSaleOfProperty	91700000	USD
CY2020Q4	us-gaap	Assets Assets	4034900000	USD
CY2019Q4	us-gaap	Assets Assets	5634600000	USD
CY2018Q4	us-gaap	Assets Assets	5501800000	USD
CY2020Q4	us-gaap	Assets Assets	4034900000	USD
CY2020Q4	ati	Entity Wide Disclosure On Geographic Areas Total Assets Attributed To Individual Countries Percentage Amount EntityWideDisclosureOnGeographicAreasTotalAssetsAttributedToIndividualCountriesPercentageAmount	1
CY2019Q4	us-gaap	Assets Assets	5634600000	USD
CY2019Q4	ati	Entity Wide Disclosure On Geographic Areas Total Assets Attributed To Individual Countries Percentage Amount EntityWideDisclosureOnGeographicAreasTotalAssetsAttributedToIndividualCountriesPercentageAmount	1
CY2018Q4	us-gaap	Assets Assets	5501800000	USD
CY2018Q4	ati	Entity Wide Disclosure On Geographic Areas Total Assets Attributed To Individual Countries Percentage Amount EntityWideDisclosureOnGeographicAreasTotalAssetsAttributedToIndividualCountriesPercentageAmount	1
CY2020	ati	Restructuringandothercharges Restructuringandothercharges	1132100000	USD
CY2020	ati	Restructuring Settlement And Impairment Provisions Excluding Goodwill Impairment Loss RestructuringSettlementAndImpairmentProvisionsExcludingGoodwillImpairmentLoss	1107500000	USD
CY2020	us-gaap	Other Asset Impairment Charges OtherAssetImpairmentCharges	1041500000	USD
CY2020	us-gaap	Severance Costs1 SeveranceCosts1	60500000	USD
CY2020	us-gaap	Severance Costs1 SeveranceCosts1	60500000	USD
CY2020	us-gaap	Restructuring And Related Cost Expected Number Of Positions Eliminated RestructuringAndRelatedCostExpectedNumberOfPositionsEliminated	1400	position
CY2020	us-gaap	Defined Benefit Plan Cost Of Providing Special Or Contractual Termination Benefit Recognized During Period DefinedBenefitPlanCostOfProvidingSpecialOrContractualTerminationBenefitRecognizedDuringPeriod	17400000	USD
CY2020	us-gaap	Inventory Write Down InventoryWriteDown	7200000	USD
CY2019	ati	Restructuringandothercharges Restructuringandothercharges	4500000	USD
CY2020	us-gaap	Goodwill Period Increase Decrease GoodwillPeriodIncreaseDecrease	-285100000	USD
CY2020	us-gaap	Goodwill Impairment Loss GoodwillImpairmentLoss	287000000.0	USD
CY2020	us-gaap	Goodwill Impairment Loss GoodwillImpairmentLoss	287000000.0	USD
CY2020	us-gaap	Goodwill Impairment Loss GoodwillImpairmentLoss	287000000.0	USD
CY2020Q4	us-gaap	Goodwill Impaired Accumulated Impairment Loss GoodwillImpairedAccumulatedImpairmentLoss	528000000.0	USD
CY2019Q4	us-gaap	Goodwill Impaired Accumulated Impairment Loss GoodwillImpairedAccumulatedImpairmentLoss	241000000.0	USD
CY2018Q4	us-gaap	Goodwill Impaired Accumulated Impairment Loss GoodwillImpairedAccumulatedImpairmentLoss	241000000.0	USD
CY2020Q4	us-gaap	Finite Lived Intangible Assets Gross FiniteLivedIntangibleAssetsGross	156200000	USD
CY2020Q4	us-gaap	Finite Lived Intangible Assets Accumulated Amortization FiniteLivedIntangibleAssetsAccumulatedAmortization	68300000	USD
CY2019Q4	us-gaap	Finite Lived Intangible Assets Gross FiniteLivedIntangibleAssetsGross	156200000	USD
CY2019Q4	us-gaap	Finite Lived Intangible Assets Accumulated Amortization FiniteLivedIntangibleAssetsAccumulatedAmortization	59900000	USD
CY2020	us-gaap	Amortization Of Intangible Assets AmortizationOfIntangibleAssets	8000000	USD
CY2019	us-gaap	Amortization Of Intangible Assets AmortizationOfIntangibleAssets	10000000	USD
CY2018	us-gaap	Amortization Of Intangible Assets AmortizationOfIntangibleAssets	10000000	USD
CY2020Q4	us-gaap	Finite Lived Intangible Assets Amortization Expense Year Five FiniteLivedIntangibleAssetsAmortizationExpenseYearFive	8000000	USD
CY2020Q4	us-gaap	Finite Lived Intangible Assets Amortization Expense Next Twelve Months FiniteLivedIntangibleAssetsAmortizationExpenseNextTwelveMonths	8000000	USD
CY2020Q4	us-gaap	Finite Lived Intangible Assets Amortization Expense Year Three FiniteLivedIntangibleAssetsAmortizationExpenseYearThree	8000000	USD
CY2020Q4	us-gaap	Finite Lived Intangible Assets Amortization Expense Year Two FiniteLivedIntangibleAssetsAmortizationExpenseYearTwo	8000000	USD
CY2020Q4	us-gaap	Finite Lived Intangible Assets Amortization Expense Year Four FiniteLivedIntangibleAssetsAmortizationExpenseYearFour	8000000	USD
CY2020	us-gaap	Number Of Operating Segments NumberOfOperatingSegments	2	segment
CY2020	us-gaap	Number Of Operating Segments NumberOfOperatingSegments	2	segment
CY2020	us-gaap	Revenue From Contract With Customer Excluding Assessed Tax RevenueFromContractWithCustomerExcludingAssessedTax	2982100000	USD
CY2019	us-gaap	Revenue From Contract With Customer Excluding Assessed Tax RevenueFromContractWithCustomerExcludingAssessedTax	4122500000	USD
CY2018	us-gaap	Revenue From Contract With Customer Excluding Assessed Tax RevenueFromContractWithCustomerExcludingAssessedTax	4046600000	USD
CY2020	us-gaap	Revenue From Contract With Customer Excluding Assessed Tax RevenueFromContractWithCustomerExcludingAssessedTax	2982100000	USD
CY2019	us-gaap	Revenue From Contract With Customer Excluding Assessed Tax RevenueFromContractWithCustomerExcludingAssessedTax	4122500000	USD
CY2018	us-gaap	Revenue From Contract With Customer Excluding Assessed Tax RevenueFromContractWithCustomerExcludingAssessedTax	4046600000	USD
CY2020	ati	Revenuefrom Contractwith Customer Excluding Assessed Tax Percentageof Revenue RevenuefromContractwithCustomerExcludingAssessedTaxPercentageofRevenue	1
CY2019	ati	Revenuefrom Contractwith Customer Excluding Assessed Tax Percentageof Revenue RevenuefromContractwithCustomerExcludingAssessedTaxPercentageofRevenue	1
CY2018	ati	Revenuefrom Contractwith Customer Excluding Assessed Tax Percentageof Revenue RevenuefromContractwithCustomerExcludingAssessedTaxPercentageofRevenue	1
CY2020Q4	us-gaap	Revenue Remaining Performance Obligation RevenueRemainingPerformanceObligation	1400000000	USD
CY2019Q4	us-gaap	Revenue Remaining Performance Obligation RevenueRemainingPerformanceObligation	2300000000	USD
CY2018Q4	us-gaap	Revenue Remaining Performance Obligation RevenueRemainingPerformanceObligation	2200000000	USD
CY2020Q4	ati	Revenue Remaining Performance Obligation Current RevenueRemainingPerformanceObligationCurrent	0.75	USD
CY2020Q4	ati	Accountsreceivablewithcustomers Accountsreceivablewithcustomers	350100000	USD
CY2019Q4	ati	Accountsreceivablewithcustomers Accountsreceivablewithcustomers	558700000	USD
CY2017Q4	us-gaap	Allowance For Doubtful Accounts Receivable AllowanceForDoubtfulAccountsReceivable	5900000	USD
CY2018	us-gaap	Provision For Doubtful Accounts ProvisionForDoubtfulAccounts	1900000	USD
CY2018	us-gaap	Allowance For Doubtful Accounts Receivable Write Offs AllowanceForDoubtfulAccountsReceivableWriteOffs	1800000	USD
CY2018Q4	us-gaap	Allowance For Doubtful Accounts Receivable AllowanceForDoubtfulAccountsReceivable	6000000.0	USD
CY2019	us-gaap	Provision For Doubtful Accounts ProvisionForDoubtfulAccounts	200000	USD
CY2019	us-gaap	Allowance For Doubtful Accounts Receivable Write Offs AllowanceForDoubtfulAccountsReceivableWriteOffs	1600000	USD
CY2019Q4	us-gaap	Allowance For Doubtful Accounts Receivable AllowanceForDoubtfulAccountsReceivable	4600000	USD
CY2020	us-gaap	Provision For Doubtful Accounts ProvisionForDoubtfulAccounts	200000	USD
CY2020	us-gaap	Allowance For Doubtful Accounts Receivable Write Offs AllowanceForDoubtfulAccountsReceivableWriteOffs	500000	USD
CY2020Q4	us-gaap	Allowance For Doubtful Accounts Receivable AllowanceForDoubtfulAccountsReceivable	4300000	USD
CY2019	ati	Contractwithcustomer Asset Current Divestiture ContractwithcustomerAssetCurrentDivestiture	7300000	USD
CY2019Q4	us-gaap	Contract With Customer Asset Net Current ContractWithCustomerAssetNetCurrent	38500000	USD
CY2018Q4	us-gaap	Contract With Customer Asset Net Current ContractWithCustomerAssetNetCurrent	51200000	USD
CY2017Q4	us-gaap	Contract With Customer Asset Net Current ContractWithCustomerAssetNetCurrent	36500000	USD
CY2020	ati	Contractwith Customer Asset Current Period Increase ContractwithCustomerAssetCurrentPeriodIncrease	84200000	USD
CY2019	ati	Contractwith Customer Asset Current Period Increase ContractwithCustomerAssetCurrentPeriodIncrease	74500000	USD
CY2018	ati	Contractwith Customer Asset Current Period Increase ContractwithCustomerAssetCurrentPeriodIncrease	92900000	USD
CY2020	ati	Contractwith Customer Asset Current Reclassifiedto Receivable ContractwithCustomerAssetCurrentReclassifiedtoReceivable	83900000	USD
CY2019	ati	Contractwith Customer Asset Current Reclassifiedto Receivable ContractwithCustomerAssetCurrentReclassifiedtoReceivable	79900000	USD
CY2018	ati	Contractwith Customer Asset Current Reclassifiedto Receivable ContractwithCustomerAssetCurrentReclassifiedtoReceivable	95800000	USD
CY2020	ati	Contractwith Customer Asset Current Impairment ContractwithCustomerAssetCurrentImpairment	0	USD
CY2019	ati	Contractwith Customer Asset Current Impairment ContractwithCustomerAssetCurrentImpairment	0	USD
CY2018	ati	Contractwith Customer Asset Current Impairment ContractwithCustomerAssetCurrentImpairment	0	USD
CY2020	ati	Contractwith Customer Asset Current Reclassification ContractwithCustomerAssetCurrentReclassification	-100000	USD
CY2019	ati	Contractwith Customer Asset Current Reclassification ContractwithCustomerAssetCurrentReclassification	0	USD
CY2018	ati	Contractwith Customer Asset Current Reclassification ContractwithCustomerAssetCurrentReclassification	-16800000	USD
CY2020	ati	Contractwithcustomer Asset Current Divestiture ContractwithcustomerAssetCurrentDivestiture	0	USD
CY2018	ati	Contractwithcustomer Asset Current Divestiture ContractwithcustomerAssetCurrentDivestiture	0	USD
CY2020	ati	Contract With Customer Asset Current Other ContractWithCustomerAssetCurrentOther	0	USD
CY2019	ati	Contract With Customer Asset Current Other ContractWithCustomerAssetCurrentOther	0	USD
CY2018	ati	Contract With Customer Asset Current Other ContractWithCustomerAssetCurrentOther	800000	USD
CY2020Q4	us-gaap	Contract With Customer Asset Net Current ContractWithCustomerAssetNetCurrent	38900000	USD
CY2019Q4	us-gaap	Contract With Customer Asset Net Current ContractWithCustomerAssetNetCurrent	38500000	USD
CY2018Q4	us-gaap	Contract With Customer Asset Net Current ContractWithCustomerAssetNetCurrent	51200000	USD
CY2019Q4	us-gaap	Contract With Customer Asset Net Noncurrent ContractWithCustomerAssetNetNoncurrent	100000	USD
CY2018Q4	us-gaap	Contract With Customer Asset Net Noncurrent ContractWithCustomerAssetNetNoncurrent	100000	USD
CY2017Q4	us-gaap	Contract With Customer Asset Net Noncurrent ContractWithCustomerAssetNetNoncurrent	16900000	USD
CY2020	ati	Contractwith Customer Asset Nocurrent Period Increase ContractwithCustomerAssetNocurrentPeriodIncrease	0	USD
CY2019	ati	Contractwith Customer Asset Nocurrent Period Increase ContractwithCustomerAssetNocurrentPeriodIncrease	0	USD
CY2018	ati	Contractwith Customer Asset Nocurrent Period Increase ContractwithCustomerAssetNocurrentPeriodIncrease	0	USD
CY2020	ati	Contractwith Customer Asset Noncurrent Reclassifiedto Receivable ContractwithCustomerAssetNoncurrentReclassifiedtoReceivable	0	USD
CY2019	ati	Contractwith Customer Asset Noncurrent Reclassifiedto Receivable ContractwithCustomerAssetNoncurrentReclassifiedtoReceivable	0	USD
CY2018	ati	Contractwith Customer Asset Noncurrent Reclassifiedto Receivable ContractwithCustomerAssetNoncurrentReclassifiedtoReceivable	0	USD
CY2020	ati	Contractwith Customer Asset Noncurrent Impairment ContractwithCustomerAssetNoncurrentImpairment	0	USD
CY2019	ati	Contractwith Customer Asset Noncurrent Impairment ContractwithCustomerAssetNoncurrentImpairment	0	USD
CY2018	ati	Contractwith Customer Asset Noncurrent Impairment ContractwithCustomerAssetNoncurrentImpairment	0	USD
CY2020	ati	Contractwith Customer Asset Noncurrent Reclassification ContractwithCustomerAssetNoncurrentReclassification	100000	USD
CY2019	ati	Contractwith Customer Asset Noncurrent Reclassification ContractwithCustomerAssetNoncurrentReclassification	0	USD
CY2018	ati	Contractwith Customer Asset Noncurrent Reclassification ContractwithCustomerAssetNoncurrentReclassification	16800000	USD
CY2020Q4	us-gaap	Contract With Customer Asset Net Noncurrent ContractWithCustomerAssetNetNoncurrent	0	USD
CY2019Q4	us-gaap	Contract With Customer Asset Net Noncurrent ContractWithCustomerAssetNetNoncurrent	100000	USD
CY2018Q4	us-gaap	Contract With Customer Asset Net Noncurrent ContractWithCustomerAssetNetNoncurrent	100000	USD
CY2019Q4	us-gaap	Contract With Customer Liability Current ContractWithCustomerLiabilityCurrent	78700000	USD
CY2018Q4	us-gaap	Contract With Customer Liability Current ContractWithCustomerLiabilityCurrent	71400000	USD
CY2017Q4	us-gaap	Contract With Customer Liability Current ContractWithCustomerLiabilityCurrent	69700000	USD
CY2020	ati	Contractwith Customer Liability Current Period Increase ContractwithCustomerLiabilityCurrentPeriodIncrease	170300000	USD
CY2019	ati	Contractwith Customer Liability Current Period Increase ContractwithCustomerLiabilityCurrentPeriodIncrease	126100000	USD
CY2018	ati	Contractwith Customer Liability Current Period Increase ContractwithCustomerLiabilityCurrentPeriodIncrease	76700000	USD
CY2020	ati	Contractwith Customer Liability Current Reclassifiedto Revenue ContractwithCustomerLiabilityCurrentReclassifiedtoRevenue	-54900000	USD
CY2019	ati	Contractwith Customer Liability Current Reclassifiedto Revenue ContractwithCustomerLiabilityCurrentReclassifiedtoRevenue	-49200000	USD
CY2018	ati	Contractwith Customer Liability Current Reclassifiedto Revenue ContractwithCustomerLiabilityCurrentReclassifiedtoRevenue	-49600000	USD
CY2020	ati	Contractwith Customer Liability Current Revenue Recognized ContractwithCustomerLiabilityCurrentRevenueRecognized	-90100000	USD
CY2019	ati	Contractwith Customer Liability Current Revenue Recognized ContractwithCustomerLiabilityCurrentRevenueRecognized	-76000000.0	USD
CY2018	ati	Contractwith Customer Liability Current Revenue Recognized ContractwithCustomerLiabilityCurrentRevenueRecognized	-42700000	USD
CY2020	ati	Contractwith Customer Liability Current Other ContractwithCustomerLiabilityCurrentOther	0	USD
CY2019	ati	Contractwith Customer Liability Current Other ContractwithCustomerLiabilityCurrentOther	1900000	USD
CY2018	ati	Contractwith Customer Liability Current Other ContractwithCustomerLiabilityCurrentOther	2700000	USD
CY2020	ati	Contractwith Customer Liability Current Reclassification ContractwithCustomerLiabilityCurrentReclassification	-7800000	USD
CY2019	ati	Contractwith Customer Liability Current Reclassification ContractwithCustomerLiabilityCurrentReclassification	-4500000	USD
CY2018	ati	Contractwith Customer Liability Current Reclassification ContractwithCustomerLiabilityCurrentReclassification	-14600000	USD
CY2020Q4	us-gaap	Contract With Customer Liability Current ContractWithCustomerLiabilityCurrent	111800000	USD
CY2019Q4	us-gaap	Contract With Customer Liability Current ContractWithCustomerLiabilityCurrent	78700000	USD
CY2018Q4	us-gaap	Contract With Customer Liability Current ContractWithCustomerLiabilityCurrent	71400000	USD
CY2019Q4	us-gaap	Contract With Customer Liability Noncurrent ContractWithCustomerLiabilityNoncurrent	25900000	USD
CY2018Q4	us-gaap	Contract With Customer Liability Noncurrent ContractWithCustomerLiabilityNoncurrent	7300000	USD
CY2017Q4	us-gaap	Contract With Customer Liability Noncurrent ContractWithCustomerLiabilityNoncurrent	22200000	USD
CY2020	ati	Contractwith Customer Liability Noncurrent Period Increase ContractwithCustomerLiabilityNoncurrentPeriodIncrease	14900000	USD
CY2019	ati	Contractwith Customer Liability Noncurrent Period Increase ContractwithCustomerLiabilityNoncurrentPeriodIncrease	24200000	USD
CY2018	ati	Contractwith Customer Liability Noncurrent Period Increase ContractwithCustomerLiabilityNoncurrentPeriodIncrease	700000	USD
CY2020	ati	Contractwith Customer Liability Noncurrent Reclassifiedto Revenue ContractwithCustomerLiabilityNoncurrentReclassifiedtoRevenue	-1000000.0	USD
CY2019	ati	Contractwith Customer Liability Noncurrent Reclassifiedto Revenue ContractwithCustomerLiabilityNoncurrentReclassifiedtoRevenue	-1100000	USD
CY2018	ati	Contractwith Customer Liability Noncurrent Reclassifiedto Revenue ContractwithCustomerLiabilityNoncurrentReclassifiedtoRevenue	-1000000.0	USD
CY2020	ati	Contractwith Customer Liability Noncurrent Revenue Recognized ContractwithCustomerLiabilityNoncurrentRevenueRecognized	0	USD
CY2019	ati	Contractwith Customer Liability Noncurrent Revenue Recognized ContractwithCustomerLiabilityNoncurrentRevenueRecognized	0	USD
CY2018	ati	Contractwith Customer Liability Noncurrent Revenue Recognized ContractwithCustomerLiabilityNoncurrentRevenueRecognized	0	USD
CY2020	ati	Contractwith Customer Liability Noncurrent Other ContractwithCustomerLiabilityNoncurrentOther	0	USD
CY2019	ati	Contractwith Customer Liability Noncurrent Other ContractwithCustomerLiabilityNoncurrentOther	0	USD
CY2018	ati	Contractwith Customer Liability Noncurrent Other ContractwithCustomerLiabilityNoncurrentOther	0	USD
CY2020	ati	Contractwith Customer Liability Noncurrent Reclassification ContractwithCustomerLiabilityNoncurrentReclassification	7800000	USD
CY2019	ati	Contractwith Customer Liability Noncurrent Reclassification ContractwithCustomerLiabilityNoncurrentReclassification	4500000	USD
CY2018	ati	Contractwith Customer Liability Noncurrent Reclassification ContractwithCustomerLiabilityNoncurrentReclassification	14600000	USD
CY2020Q4	us-gaap	Contract With Customer Liability Noncurrent ContractWithCustomerLiabilityNoncurrent	32000000.0	USD
CY2019Q4	us-gaap	Contract With Customer Liability Noncurrent ContractWithCustomerLiabilityNoncurrent	25900000	USD
CY2018Q4	us-gaap	Contract With Customer Liability Noncurrent ContractWithCustomerLiabilityNoncurrent	7300000	USD
CY2020Q4	us-gaap	Capitalized Contract Cost Net CapitalizedContractCostNet	5400000	USD
CY2019Q4	us-gaap	Capitalized Contract Cost Net CapitalizedContractCostNet	6500000	USD
CY2020	us-gaap	Capitalized Contract Cost Amortization CapitalizedContractCostAmortization	1400000	USD
CY2019	us-gaap	Capitalized Contract Cost Amortization CapitalizedContractCostAmortization	1400000	USD
CY2018	us-gaap	Capitalized Contract Cost Amortization CapitalizedContractCostAmortization	1200000	USD
CY2020Q4	us-gaap	Inventory Raw Materials And Supplies InventoryRawMaterialsAndSupplies	207600000	USD
CY2019Q4	us-gaap	Inventory Raw Materials And Supplies InventoryRawMaterialsAndSupplies	164900000	USD
CY2020Q4	us-gaap	Inventory Work In Process InventoryWorkInProcess	690700000	USD
CY2019Q4	us-gaap	Inventory Work In Process InventoryWorkInProcess	899600000	USD
CY2020Q4	us-gaap	Inventory Finished Goods InventoryFinishedGoods	181600000	USD
CY2019Q4	us-gaap	Inventory Finished Goods InventoryFinishedGoods	161300000	USD
CY2020Q4	us-gaap	Inventory Gross InventoryGross	1079900000	USD
CY2019Q4	us-gaap	Inventory Gross InventoryGross	1225800000	USD
CY2020Q4	us-gaap	Inventory Lifo Reserve InventoryLIFOReserve	-44100000	USD
CY2019Q4	us-gaap	Inventory Lifo Reserve InventoryLIFOReserve	-33600000	USD
CY2020Q4	us-gaap	Inventory Valuation Reserves InventoryValuationReserves	126900000	USD
CY2019Q4	us-gaap	Inventory Valuation Reserves InventoryValuationReserves	104100000	USD
CY2020Q4	us-gaap	Inventory Net InventoryNet	997100000	USD
CY2019Q4	us-gaap	Inventory Net InventoryNet	1155300000	USD
CY2020Q4	us-gaap	Lifo Inventory Amount LIFOInventoryAmount	627500000	USD
CY2019Q4	us-gaap	Lifo Inventory Amount LIFOInventoryAmount	776100000	USD
CY2020	us-gaap	Inventory Lifo Reserve Period Charge InventoryLIFOReservePeriodCharge	-10500000	USD
CY2019	us-gaap	Inventory Lifo Reserve Period Charge InventoryLIFOReservePeriodCharge	-25500000	USD
CY2018	us-gaap	Inventory Lifo Reserve Period Charge InventoryLIFOReservePeriodCharge	28600000	USD
CY2020	ati	Inventory Net Realizable Value Reserve Period Charge InventoryNetRealizableValueReservePeriodCharge	10500000	USD
CY2019	ati	Inventory Net Realizable Value Reserve Period Charge InventoryNetRealizableValueReservePeriodCharge	25600000	USD
CY2018	ati	Inventory Net Realizable Value Reserve Period Charge InventoryNetRealizableValueReservePeriodCharge	-27900000	USD
CY2020	ati	Lifo Provision Benefitand Changein Debit Lifonr Vreservenet LifoProvisionBenefitandChangeinDebitLIFONRVreservenet	0	USD
CY2019	ati	Lifo Provision Benefitand Changein Debit Lifonr Vreservenet LifoProvisionBenefitandChangeinDebitLIFONRVreservenet	100000	USD
CY2018	ati	Lifo Provision Benefitand Changein Debit Lifonr Vreservenet LifoProvisionBenefitandChangeinDebitLIFONRVreservenet	700000	USD
CY2020	us-gaap	Effect Of Lifo Inventory Liquidation On Income EffectOfLIFOInventoryLiquidationOnIncome	-22600000	USD
CY2019	us-gaap	Effect Of Lifo Inventory Liquidation On Income EffectOfLIFOInventoryLiquidationOnIncome	-1800000	USD
CY2018	us-gaap	Effect Of Lifo Inventory Liquidation On Income EffectOfLIFOInventoryLiquidationOnIncome	800000	USD
CY2020Q4	us-gaap	Land Land	34800000	USD
CY2019Q4	us-gaap	Land Land	34600000	USD
CY2020Q4	us-gaap	Buildings And Improvements Gross BuildingsAndImprovementsGross	564700000	USD
CY2019Q4	us-gaap	Buildings And Improvements Gross BuildingsAndImprovementsGross	832700000	USD
CY2019Q4	us-gaap	Property Plant And Equipment Gross PropertyPlantAndEquipmentGross	4538600000	USD
CY2020Q4	us-gaap	Accumulated Depreciation Depletion And Amortization Property Plant And Equipment AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment	1867200000	USD
CY2019Q2	ati	Number Of Facilities Sold NumberOfFacilitiesSold	2	facility
CY2020Q4	us-gaap	Machinery And Equipment Gross MachineryAndEquipmentGross	2736900000	USD
CY2019Q4	us-gaap	Machinery And Equipment Gross MachineryAndEquipmentGross	3671300000	USD
CY2020Q4	us-gaap	Property Plant And Equipment Gross PropertyPlantAndEquipmentGross	3336400000	USD
CY2019Q4	us-gaap	Accumulated Depreciation Depletion And Amortization Property Plant And Equipment AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment	2088500000	USD
CY2020Q4	us-gaap	Property Plant And Equipment Net PropertyPlantAndEquipmentNet	1469200000	USD
CY2019Q4	us-gaap	Property Plant And Equipment Net PropertyPlantAndEquipmentNet	2450100000	USD
CY2020Q4	us-gaap	Construction In Progress Gross ConstructionInProgressGross	233400000	USD
CY2019Q4	us-gaap	Construction In Progress Gross ConstructionInProgressGross	177300000	USD
CY2019Q2	ati	Number Of Facilities Sold NumberOfFacilitiesSold	2	facility
CY2020Q4	ati	Retained Earnings Equity Method Undistributed Earnings RetainedEarningsEquityMethodUndistributedEarnings	12700000	USD
CY2018	us-gaap	Deconsolidation Gain Or Loss Amount DeconsolidationGainOrLossAmount	15900000	USD
CY2020Q4	us-gaap	Asset Retirement Obligation AssetRetirementObligation	24000000.0	USD
CY2019Q4	us-gaap	Asset Retirement Obligation AssetRetirementObligation	23700000	USD
CY2018Q4	us-gaap	Asset Retirement Obligation AssetRetirementObligation	23100000	USD
CY2020	us-gaap	Asset Retirement Obligation Accretion Expense AssetRetirementObligationAccretionExpense	900000	USD
CY2019	us-gaap	Asset Retirement Obligation Accretion Expense AssetRetirementObligationAccretionExpense	900000	USD
CY2020	us-gaap	Asset Retirement Obligation Liabilities Settled AssetRetirementObligationLiabilitiesSettled	400000	USD
CY2019	us-gaap	Asset Retirement Obligation Liabilities Settled AssetRetirementObligationLiabilitiesSettled	300000	USD
CY2020	us-gaap	Asset Retirement Obligation Revision Of Estimate AssetRetirementObligationRevisionOfEstimate	-200000	USD
CY2019	us-gaap	Asset Retirement Obligation Revision Of Estimate AssetRetirementObligationRevisionOfEstimate	0	USD
CY2020Q4	us-gaap	Asset Retirement Obligation AssetRetirementObligation	24000000.0	USD
CY2019Q4	us-gaap	Asset Retirement Obligation AssetRetirementObligation	23700000	USD
CY2020Q4	us-gaap	Cash Cash	158200000	USD
CY2019Q4	us-gaap	Cash Cash	190800000	USD
CY2020Q4	us-gaap	Money Market Funds At Carrying Value MoneyMarketFundsAtCarryingValue	487700000	USD
CY2019Q4	us-gaap	Money Market Funds At Carrying Value MoneyMarketFundsAtCarryingValue	300000000.0	USD
CY2020Q4	us-gaap	Cash And Cash Equivalents At Carrying Value CashAndCashEquivalentsAtCarryingValue	645900000	USD
CY2019Q4	us-gaap	Cash And Cash Equivalents At Carrying Value CashAndCashEquivalentsAtCarryingValue	490800000	USD
CY2020Q4	us-gaap	Employee Related Liabilities Current EmployeeRelatedLiabilitiesCurrent	92800000	USD
CY2019Q4	us-gaap	Employee Related Liabilities Current EmployeeRelatedLiabilitiesCurrent	94500000	USD
CY2020Q4	us-gaap	Interest Payable Current InterestPayableCurrent	18000000.0	USD
CY2019Q4	us-gaap	Interest Payable Current InterestPayableCurrent	25000000.0	USD
CY2020	ati	Rent Royalty And Other Income RentRoyaltyAndOtherIncome	900000	USD
CY2019	ati	Rent Royalty And Other Income RentRoyaltyAndOtherIncome	2900000	USD
CY2018	ati	Rent Royalty And Other Income RentRoyaltyAndOtherIncome	3100000	USD
CY2020	us-gaap	Gain Loss On Disposition Of Assets1 GainLossOnDispositionOfAssets1	2900000	USD
CY2019	us-gaap	Gain Loss On Disposition Of Assets1 GainLossOnDispositionOfAssets1	90700000	USD
CY2018	us-gaap	Gain Loss On Disposition Of Assets1 GainLossOnDispositionOfAssets1	1300000	USD
CY2020	us-gaap	Income Loss From Equity Method Investments IncomeLossFromEquityMethodInvestments	-7000000.0	USD
CY2019	us-gaap	Income Loss From Equity Method Investments IncomeLossFromEquityMethodInvestments	-10700000	USD
CY2018	us-gaap	Income Loss From Equity Method Investments IncomeLossFromEquityMethodInvestments	-1000000.0	USD
CY2020	us-gaap	Gain Loss On Sale Of Business GainLossOnSaleOfBusiness	0	USD
CY2019	us-gaap	Gain Loss On Sale Of Business GainLossOnSaleOfBusiness	-1900000	USD
CY2018	us-gaap	Gain Loss On Sale Of Business GainLossOnSaleOfBusiness	0	USD
CY2020	us-gaap	Deconsolidation Gain Or Loss Amount DeconsolidationGainOrLossAmount	0	USD
CY2019	us-gaap	Deconsolidation Gain Or Loss Amount DeconsolidationGainOrLossAmount	0	USD
CY2018	us-gaap	Deconsolidation Gain Or Loss Amount DeconsolidationGainOrLossAmount	15900000	USD
CY2020	ati	Other Asset Impairment Charges And Due From Affiliates Allowance For Credit Loss OtherAssetImpairmentChargesAndDueFromAffiliatesAllowanceForCreditLoss	2400000	USD
CY2019	ati	Other Asset Impairment Charges And Due From Affiliates Allowance For Credit Loss OtherAssetImpairmentChargesAndDueFromAffiliatesAllowanceForCreditLoss	11400000	USD
CY2018	ati	Other Asset Impairment Charges And Due From Affiliates Allowance For Credit Loss OtherAssetImpairmentChargesAndDueFromAffiliatesAllowanceForCreditLoss	0	USD
CY2020	ati	Other Other Nonoperating Income Expense OtherOtherNonoperatingIncomeExpense	100000	USD
CY2019	ati	Other Other Nonoperating Income Expense OtherOtherNonoperatingIncomeExpense	100000	USD
CY2018	ati	Other Other Nonoperating Income Expense OtherOtherNonoperatingIncomeExpense	1200000	USD
CY2020	us-gaap	Other Nonoperating Income Expense OtherNonoperatingIncomeExpense	-1200000	USD
CY2019	us-gaap	Other Nonoperating Income Expense OtherNonoperatingIncomeExpense	69700000	USD
CY2018	us-gaap	Other Nonoperating Income Expense OtherNonoperatingIncomeExpense	20500000	USD
CY2020	us-gaap	Gain Loss On Sales Of Assets And Asset Impairment Charges GainLossOnSalesOfAssetsAndAssetImpairmentCharges	2500000	USD
CY2019	us-gaap	Gain Loss On Sale Of Property GainLossOnSaleOfProperty	91700000	USD
CY2020Q4	us-gaap	Deferred Finance Costs Net DeferredFinanceCostsNet	14500000	USD
CY2019Q4	us-gaap	Deferred Finance Costs Net DeferredFinanceCostsNet	12300000	USD
CY2020Q4	ati	Total Debt TotalDebt	1567800000	USD
CY2019Q4	ati	Total Debt TotalDebt	1398900000	USD
CY2020Q4	us-gaap	Debt Current DebtCurrent	17800000	USD
CY2019Q4	us-gaap	Debt Current DebtCurrent	11500000	USD
CY2020Q4	us-gaap	Long Term Debt And Capital Lease Obligations LongTermDebtAndCapitalLeaseObligations	1550000000.0	USD
CY2019Q4	us-gaap	Long Term Debt And Capital Lease Obligations LongTermDebtAndCapitalLeaseObligations	1387400000	USD
CY2020	us-gaap	Interest Expense Debt InterestExpenseDebt	96100000	USD
CY2019	us-gaap	Interest Expense Debt InterestExpenseDebt	104900000	USD
CY2018	us-gaap	Interest Expense Debt InterestExpenseDebt	102100000	USD
CY2020	us-gaap	Interest Costs Capitalized InterestCostsCapitalized	7700000	USD
CY2019	us-gaap	Interest Costs Capitalized InterestCostsCapitalized	4700000	USD
CY2018	us-gaap	Interest Costs Capitalized InterestCostsCapitalized	4100000	USD
CY2020	us-gaap	Interest Paid InterestPaid	95400000	USD
CY2019	us-gaap	Interest Paid InterestPaid	105700000	USD
CY2018	us-gaap	Interest Paid InterestPaid	102600000	USD
CY2021Q1	ati	Progress Payments Made On Behalf Of Company Scheduled To Be Paid ProgressPaymentsMadeOnBehalfOfCompanyScheduledToBePaid	29000000.0	USD
CY2020	us-gaap	Investment Income Interest InvestmentIncomeInterest	1700000	USD
CY2019	us-gaap	Investment Income Interest InvestmentIncomeInterest	5900000	USD
CY2018	us-gaap	Investment Income Interest InvestmentIncomeInterest	1100000	USD
CY2020Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Next Twelve Months LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths	17800000	USD
CY2020Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Year Two LongTermDebtMaturitiesRepaymentsOfPrincipalInYearTwo	95900000	USD
CY2020Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Year Three LongTermDebtMaturitiesRepaymentsOfPrincipalInYearThree	511000000.0	USD
CY2020Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Year Four LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFour	208100000	USD
CY2020Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Year Five LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFive	446100000	USD
CY2020	us-gaap	Gains Losses On Extinguishment Of Debt GainsLossesOnExtinguishmentOfDebt	-21500000	USD
CY2020	us-gaap	Payments Of Debt Extinguishment Costs PaymentsOfDebtExtinguishmentCosts	19100000	USD
CY2019	us-gaap	Gains Losses On Extinguishment Of Debt GainsLossesOnExtinguishmentOfDebt	-21600000	USD
CY2019	us-gaap	Payments Of Debt Extinguishment Costs PaymentsOfDebtExtinguishmentCosts	20900000	USD
CY2020	us-gaap	Gains Losses On Extinguishment Of Debt GainsLossesOnExtinguishmentOfDebt	-21500000	USD
CY2020	ati	Payment Of Capped Call Transactions Cost PaymentOfCappedCallTransactionsCost	19400000	USD
CY2019	us-gaap	Payments Of Debt Issuance Costs PaymentsOfDebtIssuanceCosts	5500000	USD
CY2020Q4	ati	Progress Payments Made On Behalf Of Company Paid ProgressPaymentsMadeOnBehalfOfCompanyPaid	17200000	USD
CY2020	us-gaap	Finance Lease Right Of Use Asset Amortization FinanceLeaseRightOfUseAssetAmortization	3500000	USD
CY2019	us-gaap	Finance Lease Right Of Use Asset Amortization FinanceLeaseRightOfUseAssetAmortization	1700000	USD
CY2020	us-gaap	Finance Lease Interest Expense FinanceLeaseInterestExpense	1000000.0	USD
CY2019	us-gaap	Finance Lease Interest Expense FinanceLeaseInterestExpense	500000	USD
CY2020	us-gaap	Operating Lease Cost OperatingLeaseCost	20800000	USD
CY2019	us-gaap	Operating Lease Cost OperatingLeaseCost	20500000	USD
CY2020	us-gaap	Short Term Lease Cost ShortTermLeaseCost	1900000	USD
CY2019	us-gaap	Short Term Lease Cost ShortTermLeaseCost	3100000	USD
CY2020	us-gaap	Variable Lease Cost VariableLeaseCost	900000	USD
CY2019	us-gaap	Variable Lease Cost VariableLeaseCost	800000	USD
CY2020	us-gaap	Sublease Income SubleaseIncome	100000	USD
CY2019	us-gaap	Sublease Income SubleaseIncome	0	USD
CY2020	us-gaap	Lease Cost LeaseCost	28000000.0	USD
CY2019	us-gaap	Lease Cost LeaseCost	26600000	USD
CY2020	us-gaap	Finance Lease Interest Payment On Liability FinanceLeaseInterestPaymentOnLiability	1000000.0	USD
CY2019	us-gaap	Finance Lease Interest Payment On Liability FinanceLeaseInterestPaymentOnLiability	500000	USD
CY2020	us-gaap	Operating Lease Payments OperatingLeasePayments	21700000	USD
CY2019	us-gaap	Operating Lease Payments OperatingLeasePayments	20800000	USD
CY2020	us-gaap	Finance Lease Principal Payments FinanceLeasePrincipalPayments	6200000	USD
CY2019	us-gaap	Finance Lease Principal Payments FinanceLeasePrincipalPayments	2400000	USD
CY2020	us-gaap	Right Of Use Asset Obtained In Exchange For Finance Lease Liability RightOfUseAssetObtainedInExchangeForFinanceLeaseLiability	42300000	USD
CY2019	us-gaap	Right Of Use Asset Obtained In Exchange For Finance Lease Liability RightOfUseAssetObtainedInExchangeForFinanceLeaseLiability	14100000	USD
CY2020	us-gaap	Right Of Use Asset Obtained In Exchange For Operating Lease Liability RightOfUseAssetObtainedInExchangeForOperatingLeaseLiability	12400000	USD
CY2019	us-gaap	Right Of Use Asset Obtained In Exchange For Operating Lease Liability RightOfUseAssetObtainedInExchangeForOperatingLeaseLiability	35900000	USD
CY2020Q4	us-gaap	Finance Lease Weighted Average Remaining Lease Term1 FinanceLeaseWeightedAverageRemainingLeaseTerm1	P4Y
CY2019Q4	us-gaap	Finance Lease Weighted Average Remaining Lease Term1 FinanceLeaseWeightedAverageRemainingLeaseTerm1	P4Y
CY2020Q4	us-gaap	Operating Lease Weighted Average Remaining Lease Term1 OperatingLeaseWeightedAverageRemainingLeaseTerm1	P6Y
CY2019Q4	us-gaap	Operating Lease Weighted Average Remaining Lease Term1 OperatingLeaseWeightedAverageRemainingLeaseTerm1	P6Y
CY2020Q4	us-gaap	Finance Lease Weighted Average Discount Rate Percent FinanceLeaseWeightedAverageDiscountRatePercent	0.062
CY2019Q4	us-gaap	Finance Lease Weighted Average Discount Rate Percent FinanceLeaseWeightedAverageDiscountRatePercent	0.053
CY2020Q4	us-gaap	Operating Lease Weighted Average Discount Rate Percent OperatingLeaseWeightedAverageDiscountRatePercent	0.069
CY2019Q4	us-gaap	Operating Lease Weighted Average Discount Rate Percent OperatingLeaseWeightedAverageDiscountRatePercent	0.070
CY2018	us-gaap	Lease And Rental Expense LeaseAndRentalExpense	24400000	USD
CY2020Q4	us-gaap	Lessee Operating Lease Liability Payments Due Next Twelve Months LesseeOperatingLeaseLiabilityPaymentsDueNextTwelveMonths	19600000	USD
CY2020Q4	us-gaap	Lessee Operating Lease Liability Payments Due Year Two LesseeOperatingLeaseLiabilityPaymentsDueYearTwo	16600000	USD
CY2020Q4	us-gaap	Lessee Operating Lease Liability Payments Due Year Three LesseeOperatingLeaseLiabilityPaymentsDueYearThree	12800000	USD
CY2020Q4	us-gaap	Lessee Operating Lease Liability Payments Due Year Four LesseeOperatingLeaseLiabilityPaymentsDueYearFour	9300000	USD
CY2020Q4	us-gaap	Lessee Operating Lease Liability Payments Due Year Five LesseeOperatingLeaseLiabilityPaymentsDueYearFive	7600000	USD
CY2020Q4	us-gaap	Lessee Operating Lease Liability Payments Due After Year Five LesseeOperatingLeaseLiabilityPaymentsDueAfterYearFive	18100000	USD
CY2020Q4	us-gaap	Lessee Operating Lease Liability Payments Due LesseeOperatingLeaseLiabilityPaymentsDue	84000000.0	USD
CY2020Q4	us-gaap	Lessee Operating Lease Liability Undiscounted Excess Amount LesseeOperatingLeaseLiabilityUndiscountedExcessAmount	16100000	USD
CY2020Q4	us-gaap	Operating Lease Liability OperatingLeaseLiability	67900000	USD
CY2020Q4	us-gaap	Finance Lease Liability Payments Due Next Twelve Months FinanceLeaseLiabilityPaymentsDueNextTwelveMonths	13800000	USD
CY2020Q4	us-gaap	Finance Lease Liability Undiscounted Excess Amount FinanceLeaseLiabilityUndiscountedExcessAmount	6400000	USD
CY2020Q4	us-gaap	Finance Lease Liability Payments Due Year Two FinanceLeaseLiabilityPaymentsDueYearTwo	13200000	USD
CY2020Q4	us-gaap	Finance Lease Liability Payments Due Year Three FinanceLeaseLiabilityPaymentsDueYearThree	12200000	USD
CY2020Q4	us-gaap	Finance Lease Liability Payments Due Year Four FinanceLeaseLiabilityPaymentsDueYearFour	8700000	USD
CY2020Q4	us-gaap	Finance Lease Liability Payments Due Year Five FinanceLeaseLiabilityPaymentsDueYearFive	4800000	USD
CY2020Q4	us-gaap	Finance Lease Liability Payments Due After Year Five FinanceLeaseLiabilityPaymentsDueAfterYearFive	100000	USD
CY2020Q4	us-gaap	Finance Lease Liability Payments Due FinanceLeaseLiabilityPaymentsDue	52800000	USD
CY2020Q4	us-gaap	Finance Lease Liability FinanceLeaseLiability	46400000	USD
CY2020	ati	Percentage Of Raw Material Usage Hedged PercentageOfRawMaterialUsageHedged	0.05
CY2020Q4	us-gaap	Derivative Fair Value Of Derivative Asset DerivativeFairValueOfDerivativeAsset	4800000	USD
CY2019Q4	us-gaap	Derivative Fair Value Of Derivative Asset DerivativeFairValueOfDerivativeAsset	5600000	USD
CY2020Q4	us-gaap	Derivative Fair Value Of Derivative Liability DerivativeFairValueOfDerivativeLiability	4000000.0	USD
CY2019Q4	us-gaap	Derivative Fair Value Of Derivative Liability DerivativeFairValueOfDerivativeLiability	7500000	USD
CY2020	us-gaap	Cash Flow Hedge Gain Loss To Be Reclassified Within Twelve Months CashFlowHedgeGainLossToBeReclassifiedWithinTwelveMonths	2500000	USD
CY2019Q4	us-gaap	Other Comprehensive Income Loss Cash Flow Hedge Gain Loss Reclassification Tax OtherComprehensiveIncomeLossCashFlowHedgeGainLossReclassificationTax	6000000.0	USD
CY2020Q4	ati	Defined Contribution Plan Deferred Contributions Due To Covid19 Pandemic DefinedContributionPlanDeferredContributionsDueToCOVID19Pandemic	7300000	USD
CY2020	us-gaap	Defined Contribution Plan Cost Recognized DefinedContributionPlanCostRecognized	29900000	USD
CY2019	us-gaap	Defined Contribution Plan Cost Recognized DefinedContributionPlanCostRecognized	44800000	USD
CY2018	us-gaap	Defined Contribution Plan Cost Recognized DefinedContributionPlanCostRecognized	39900000	USD
CY2020	us-gaap	Defined Benefit Plan Cost Of Providing Special Or Contractual Termination Benefit Recognized During Period DefinedBenefitPlanCostOfProvidingSpecialOrContractualTerminationBenefitRecognizedDuringPeriod	17400000	USD
CY2020	us-gaap	Other Comprehensive Income Loss Before Reclassifications Net Of Tax OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax	-80200000	USD
CY2020	ati	Projected Retirement Benefit Expense In Next Year ProjectedRetirementBenefitExpenseInNextYear	44000000	USD
CY2020Q4	us-gaap	Defined Benefit Plan Expected Amortization Of Prior Service Cost Credit Next Fiscal Year DefinedBenefitPlanExpectedAmortizationOfPriorServiceCostCreditNextFiscalYear	-1900000	USD
CY2020Q4	us-gaap	Defined Benefit Plan Expected Amortization Of Gain Loss Next Fiscal Year DefinedBenefitPlanExpectedAmortizationOfGainLossNextFiscalYear	-88800000	USD
CY2020Q4	us-gaap	Defined Benefit Plan Expected Amortization Next Fiscal Year DefinedBenefitPlanExpectedAmortizationNextFiscalYear	86900000	USD
CY2020	ati	Multiemployer Plans Withdrawal Obligation Payment Period MultiemployerPlansWithdrawalObligationPaymentPeriod	P20Y
CY2017Q4	us-gaap	Accumulated Other Comprehensive Income Loss Net Of Tax AccumulatedOtherComprehensiveIncomeLossNetOfTax	-1027800000	USD
CY2018	us-gaap	Other Comprehensive Income Loss Before Reclassifications Net Of Tax OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax	-132500000	USD
CY2018	us-gaap	Reclassification From Accumulated Other Comprehensive Income Current Period Net Of Tax ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax	-26500000	USD
CY2018	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	-106000000.0	USD
CY2018Q4	us-gaap	Accumulated Other Comprehensive Income Loss Net Of Tax AccumulatedOtherComprehensiveIncomeLossNetOfTax	-1133800000	USD
CY2019	us-gaap	Other Comprehensive Income Loss Before Reclassifications Net Of Tax OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax	-136900000	USD
CY2019	us-gaap	Reclassification From Accumulated Other Comprehensive Income Current Period Net Of Tax ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax	-69000000.0	USD
CY2019	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	-67900000	USD
CY2019Q4	us-gaap	Accumulated Other Comprehensive Income Loss Net Of Tax AccumulatedOtherComprehensiveIncomeLossNetOfTax	-1201700000	USD
CY2017Q4	ati	Accumulated Other Comprehensive Income Loss Netof Tax Attributableto Noncontrolling Interest AccumulatedOtherComprehensiveIncomeLossNetofTaxAttributabletoNoncontrollingInterest	17300000	USD
CY2018	ati	Other Comprehensive Income Lossbefore Reclassifications Netof Tax Attributableto Noncontrolling Interest OtherComprehensiveIncomeLossbeforeReclassificationsNetofTaxAttributabletoNoncontrollingInterest	-6200000	USD
CY2020	us-gaap	Reclassification From Accumulated Other Comprehensive Income Current Period Net Of Tax ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax	-58300000	USD
CY2020	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	-21900000	USD
CY2020Q4	us-gaap	Accumulated Other Comprehensive Income Loss Net Of Tax AccumulatedOtherComprehensiveIncomeLossNetOfTax	-1223600000	USD
CY2018	ati	Reclassificationfrom Accumulated Other Comprehensive Income Current Period Netof Tax Attributableto Noncontrolling Interest ReclassificationfromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetofTaxAttributabletoNoncontrollingInterest	0	USD
CY2018	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Noncontrolling Interest OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToNoncontrollingInterest	-6200000	USD
CY2018Q4	ati	Accumulated Other Comprehensive Income Loss Netof Tax Attributableto Noncontrolling Interest AccumulatedOtherComprehensiveIncomeLossNetofTaxAttributabletoNoncontrollingInterest	11100000	USD
CY2019	ati	Other Comprehensive Income Lossbefore Reclassifications Netof Tax Attributableto Noncontrolling Interest OtherComprehensiveIncomeLossbeforeReclassificationsNetofTaxAttributabletoNoncontrollingInterest	-1300000	USD
CY2019	ati	Reclassificationfrom Accumulated Other Comprehensive Income Current Period Netof Tax Attributableto Noncontrolling Interest ReclassificationfromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetofTaxAttributabletoNoncontrollingInterest	0	USD
CY2019	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Noncontrolling Interest OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToNoncontrollingInterest	-1300000	USD
CY2019Q4	ati	Accumulated Other Comprehensive Income Loss Netof Tax Attributableto Noncontrolling Interest AccumulatedOtherComprehensiveIncomeLossNetofTaxAttributabletoNoncontrollingInterest	9800000	USD
CY2020	ati	Other Comprehensive Income Lossbefore Reclassifications Netof Tax Attributableto Noncontrolling Interest OtherComprehensiveIncomeLossbeforeReclassificationsNetofTaxAttributabletoNoncontrollingInterest	11400000	USD
CY2020	ati	Reclassificationfrom Accumulated Other Comprehensive Income Current Period Netof Tax Attributableto Noncontrolling Interest ReclassificationfromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetofTaxAttributabletoNoncontrollingInterest	0	USD
CY2020	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Noncontrolling Interest OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToNoncontrollingInterest	11400000	USD
CY2020Q4	ati	Accumulated Other Comprehensive Income Loss Netof Tax Attributableto Noncontrolling Interest AccumulatedOtherComprehensiveIncomeLossNetofTaxAttributabletoNoncontrollingInterest	21200000	USD
CY2018	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Grants In Period ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod	290000	shares
CY2019Q4	us-gaap	Other Comprehensive Income Loss Cash Flow Hedge Gain Loss Reclassification Tax OtherComprehensiveIncomeLossCashFlowHedgeGainLossReclassificationTax	6000000.0	USD
CY2018	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Grants In Period Weighted Average Grant Date Fair Value ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue	7800000
CY2020	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Vested In Period ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod	450000	shares
CY2020Q4	us-gaap	Preferred Stock Shares Issued PreferredStockSharesIssued	0	shares
CY2020Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Number Of Shares Available For Grant ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant	5300000	shares
CY2019Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Number ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber	756000	shares
CY2019Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Weighted Average Grant Date Fair Value ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue	19600000
CY2018Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Number ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber	1055000	shares
CY2018Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Weighted Average Grant Date Fair Value ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue	23700000
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CY2017Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Weighted Average Grant Date Fair Value ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue	27900000
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CY2020	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Grants In Period Weighted Average Grant Date Fair Value ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue	10100000
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CY2019	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Grants In Period Weighted Average Grant Date Fair Value ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue	10800000
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CY2019	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Vested In Period Weighted Average Grant Date Fair Value ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue	14500000
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CY2018	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Vested In Period Weighted Average Grant Date Fair Value ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue	11700000
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CY2020	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Forfeitures Weighted Average Grant Date Fair Value ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue	600000
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CY2019	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Forfeitures Weighted Average Grant Date Fair Value ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue	400000
CY2018	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Forfeited In Period ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod	15000	shares
CY2018	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Forfeitures Weighted Average Grant Date Fair Value ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue	300000
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CY2020Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Weighted Average Grant Date Fair Value ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue	17900000
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CY2019Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Weighted Average Grant Date Fair Value ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue	19600000
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CY2018Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Weighted Average Grant Date Fair Value ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue	23700000
CY2017Q4	ati	Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Weighted Average Grant Date Fair Value Reduction Due To Prior Year Correction ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueReductionDueToPriorYearCorrection	4600000	USD
CY2020	us-gaap	Income Loss From Continuing Operations Before Income Taxes Domestic IncomeLossFromContinuingOperationsBeforeIncomeTaxesDomestic	-1505400000	USD
CY2019	us-gaap	Income Loss From Continuing Operations Before Income Taxes Domestic IncomeLossFromContinuingOperationsBeforeIncomeTaxesDomestic	190200000	USD
CY2018	us-gaap	Income Loss From Continuing Operations Before Income Taxes Domestic IncomeLossFromContinuingOperationsBeforeIncomeTaxesDomestic	190800000	USD
CY2020	us-gaap	Income Loss From Continuing Operations Before Income Taxes Foreign IncomeLossFromContinuingOperationsBeforeIncomeTaxesForeign	23500000	USD
CY2019	us-gaap	Income Loss From Continuing Operations Before Income Taxes Foreign IncomeLossFromContinuingOperationsBeforeIncomeTaxesForeign	51400000	USD
CY2018	us-gaap	Income Loss From Continuing Operations Before Income Taxes Foreign IncomeLossFromContinuingOperationsBeforeIncomeTaxesForeign	56900000	USD
CY2020	us-gaap	Income Loss From Continuing Operations Before Income Taxes Extraordinary Items Noncontrolling Interest IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	-1481900000	USD
CY2019	us-gaap	Income Loss From Continuing Operations Before Income Taxes Extraordinary Items Noncontrolling Interest IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	241600000	USD
CY2018	us-gaap	Income Loss From Continuing Operations Before Income Taxes Extraordinary Items Noncontrolling Interest IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	247700000	USD
CY2020	us-gaap	Current Federal Tax Expense Benefit CurrentFederalTaxExpenseBenefit	600000	USD
CY2019	us-gaap	Current Federal Tax Expense Benefit CurrentFederalTaxExpenseBenefit	2200000	USD
CY2018	us-gaap	Current Federal Tax Expense Benefit CurrentFederalTaxExpenseBenefit	1000000.0	USD
CY2020	us-gaap	Current State And Local Tax Expense Benefit CurrentStateAndLocalTaxExpenseBenefit	-1100000	USD
CY2019	us-gaap	Current State And Local Tax Expense Benefit CurrentStateAndLocalTaxExpenseBenefit	200000	USD
CY2018	us-gaap	Current State And Local Tax Expense Benefit CurrentStateAndLocalTaxExpenseBenefit	-800000	USD
CY2020	us-gaap	Current Foreign Tax Expense Benefit CurrentForeignTaxExpenseBenefit	6700000	USD
CY2019	us-gaap	Current Foreign Tax Expense Benefit CurrentForeignTaxExpenseBenefit	8100000	USD
CY2018	us-gaap	Current Foreign Tax Expense Benefit CurrentForeignTaxExpenseBenefit	10100000	USD
CY2020	us-gaap	Current Income Tax Expense Benefit CurrentIncomeTaxExpenseBenefit	6200000	USD
CY2019	us-gaap	Current Income Tax Expense Benefit CurrentIncomeTaxExpenseBenefit	10500000	USD
CY2018	us-gaap	Current Income Tax Expense Benefit CurrentIncomeTaxExpenseBenefit	10300000	USD
CY2020	us-gaap	Deferred Federal Income Tax Expense Benefit DeferredFederalIncomeTaxExpenseBenefit	26600000	USD
CY2019	us-gaap	Deferred Federal Income Tax Expense Benefit DeferredFederalIncomeTaxExpenseBenefit	-4600000	USD
CY2018	us-gaap	Deferred Federal Income Tax Expense Benefit DeferredFederalIncomeTaxExpenseBenefit	1300000	USD
CY2020	us-gaap	Deferred State And Local Income Tax Expense Benefit DeferredStateAndLocalIncomeTaxExpenseBenefit	47100000	USD
CY2019	us-gaap	Deferred State And Local Income Tax Expense Benefit DeferredStateAndLocalIncomeTaxExpenseBenefit	-40400000	USD
CY2018	us-gaap	Deferred State And Local Income Tax Expense Benefit DeferredStateAndLocalIncomeTaxExpenseBenefit	-500000	USD
CY2020	us-gaap	Deferred Foreign Income Tax Expense Benefit DeferredForeignIncomeTaxExpenseBenefit	-2200000	USD
CY2019	us-gaap	Deferred Foreign Income Tax Expense Benefit DeferredForeignIncomeTaxExpenseBenefit	6000000.0	USD
CY2018	us-gaap	Deferred Foreign Income Tax Expense Benefit DeferredForeignIncomeTaxExpenseBenefit	-100000	USD
CY2020	us-gaap	Deferred Income Tax Expense Benefit DeferredIncomeTaxExpenseBenefit	71500000	USD
CY2019	us-gaap	Deferred Income Tax Expense Benefit DeferredIncomeTaxExpenseBenefit	-39000000.0	USD
CY2018	us-gaap	Deferred Income Tax Expense Benefit DeferredIncomeTaxExpenseBenefit	700000	USD
CY2020	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	77700000	USD
CY2019	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	-28500000	USD
CY2018	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	11000000.0	USD
CY2020	us-gaap	Income Tax Reconciliation Income Tax Expense Benefit At Federal Statutory Income Tax Rate IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate	-311200000	USD
CY2019	us-gaap	Income Tax Reconciliation Income Tax Expense Benefit At Federal Statutory Income Tax Rate IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate	50700000	USD
CY2018	us-gaap	Income Tax Reconciliation Income Tax Expense Benefit At Federal Statutory Income Tax Rate IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate	52000000.0	USD
CY2020	us-gaap	Income Tax Reconciliation Nondeductible Expense Impairment Losses IncomeTaxReconciliationNondeductibleExpenseImpairmentLosses	50400000	USD
CY2019	us-gaap	Income Tax Reconciliation Nondeductible Expense Impairment Losses IncomeTaxReconciliationNondeductibleExpenseImpairmentLosses	0	USD
CY2018	us-gaap	Income Tax Reconciliation Nondeductible Expense Impairment Losses IncomeTaxReconciliationNondeductibleExpenseImpairmentLosses	0	USD
CY2020	us-gaap	Income Tax Reconciliation State And Local Income Taxes IncomeTaxReconciliationStateAndLocalIncomeTaxes	-200000	USD
CY2019	us-gaap	Income Tax Reconciliation State And Local Income Taxes IncomeTaxReconciliationStateAndLocalIncomeTaxes	300000	USD
CY2018	us-gaap	Income Tax Reconciliation State And Local Income Taxes IncomeTaxReconciliationStateAndLocalIncomeTaxes	-500000	USD
CY2020	us-gaap	Income Tax Reconciliation Change In Deferred Tax Assets Valuation Allowance IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance	335500000	USD
CY2019	us-gaap	Income Tax Reconciliation Change In Deferred Tax Assets Valuation Allowance IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance	-90100000	USD
CY2018	us-gaap	Income Tax Reconciliation Change In Deferred Tax Assets Valuation Allowance IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance	-48000000.0	USD
CY2020	us-gaap	Income Tax Reconciliation Repatriation Of Foreign Earnings IncomeTaxReconciliationRepatriationOfForeignEarnings	200000	USD
CY2019	us-gaap	Income Tax Reconciliation Repatriation Of Foreign Earnings IncomeTaxReconciliationRepatriationOfForeignEarnings	3500000	USD
CY2018	us-gaap	Income Tax Reconciliation Repatriation Of Foreign Earnings IncomeTaxReconciliationRepatriationOfForeignEarnings	5400000	USD
CY2020	ati	Effective Income Tax Rate Reconciliation Restructuring EffectiveIncomeTaxRateReconciliationRestructuring	0	USD
CY2019	ati	Effective Income Tax Rate Reconciliation Restructuring EffectiveIncomeTaxRateReconciliationRestructuring	4200000	USD
CY2018	ati	Effective Income Tax Rate Reconciliation Restructuring EffectiveIncomeTaxRateReconciliationRestructuring	0	USD
CY2020	ati	Income Tax Reconciliation U.S.Taxreform IncomeTaxReconciliationU.S.taxreform	0	USD
CY2019	ati	Income Tax Reconciliation U.S.Taxreform IncomeTaxReconciliationU.S.taxreform	0	USD
CY2018	ati	Income Tax Reconciliation U.S.Taxreform IncomeTaxReconciliationU.S.taxreform	5900000	USD
CY2020	us-gaap	Income Tax Reconciliation Foreign Income Tax Rate Differential IncomeTaxReconciliationForeignIncomeTaxRateDifferential	1700000	USD
CY2019	us-gaap	Income Tax Reconciliation Foreign Income Tax Rate Differential IncomeTaxReconciliationForeignIncomeTaxRateDifferential	2700000	USD
CY2018	us-gaap	Income Tax Reconciliation Foreign Income Tax Rate Differential IncomeTaxReconciliationForeignIncomeTaxRateDifferential	3200000	USD
CY2020	us-gaap	Income Tax Reconciliation Prior Year Income Taxes IncomeTaxReconciliationPriorYearIncomeTaxes	0	USD
CY2019	us-gaap	Income Tax Reconciliation Prior Year Income Taxes IncomeTaxReconciliationPriorYearIncomeTaxes	0	USD
CY2018	us-gaap	Income Tax Reconciliation Prior Year Income Taxes IncomeTaxReconciliationPriorYearIncomeTaxes	-5800000	USD
CY2020	ati	Effective Income Tax Rate Reconciliation Withholding Taxes EffectiveIncomeTaxRateReconciliationWithholdingTaxes	2100000	USD
CY2019	ati	Effective Income Tax Rate Reconciliation Withholding Taxes EffectiveIncomeTaxRateReconciliationWithholdingTaxes	2700000	USD
CY2018	ati	Effective Income Tax Rate Reconciliation Withholding Taxes EffectiveIncomeTaxRateReconciliationWithholdingTaxes	2700000	USD
CY2020	us-gaap	Income Tax Reconciliation Tax Holidays IncomeTaxReconciliationTaxHolidays	4600000	USD
CY2019	us-gaap	Income Tax Reconciliation Tax Holidays IncomeTaxReconciliationTaxHolidays	4100000	USD
CY2018	us-gaap	Income Tax Reconciliation Tax Holidays IncomeTaxReconciliationTaxHolidays	4800000	USD
CY2020	us-gaap	Income Tax Reconciliation Other Reconciling Items IncomeTaxReconciliationOtherReconcilingItems	3800000	USD
CY2019	us-gaap	Income Tax Reconciliation Other Reconciling Items IncomeTaxReconciliationOtherReconcilingItems	1600000	USD
CY2018	us-gaap	Income Tax Reconciliation Other Reconciling Items IncomeTaxReconciliationOtherReconcilingItems	900000	USD
CY2020	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	77700000	USD
CY2019	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	-28500000	USD
CY2018	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	11000000.0	USD
CY2020	us-gaap	Goodwill Impairment Loss GoodwillImpairmentLoss	287000000.0	USD
CY2020	us-gaap	Income Tax Reconciliation Nondeductible Expense Impairment Losses IncomeTaxReconciliationNondeductibleExpenseImpairmentLosses	50400000	USD
CY2020	us-gaap	Income Tax Reconciliation Change In Deferred Tax Assets Valuation Allowance IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance	335500000	USD
CY2019	us-gaap	Valuation Allowance Deferred Tax Asset Change In Amount ValuationAllowanceDeferredTaxAssetChangeInAmount	-45100000	USD
CY2019Q4	us-gaap	Other Comprehensive Income Loss Cash Flow Hedge Gain Loss Reclassification Tax OtherComprehensiveIncomeLossCashFlowHedgeGainLossReclassificationTax	6000000.0	USD
CY2018	ati	Tax Cutsand Jobs Act Transition Taxfor Accumulated Foreign Earnings Income Tax Expense TaxCutsandJobsActTransitionTaxforAccumulatedForeignEarningsIncomeTaxExpense	5900000	USD
CY2019	ati	Tax Cuts And Jobs Act Foreign Earnings Inclusion Global Intangible Low Taxed Income Gilti TaxCutsAndJobsActForeignEarningsInclusionGlobalIntangibleLowTaxedIncomeGILTI	16800000	USD
CY2019	us-gaap	Income Tax Reconciliation Repatriation Of Foreign Earnings IncomeTaxReconciliationRepatriationOfForeignEarnings	3500000	USD
CY2018	ati	Tax Cuts And Jobs Act Foreign Earnings Inclusion Global Intangible Low Taxed Income Gilti TaxCutsAndJobsActForeignEarningsInclusionGlobalIntangibleLowTaxedIncomeGILTI	25700000	USD
CY2018	us-gaap	Income Tax Reconciliation Repatriation Of Foreign Earnings IncomeTaxReconciliationRepatriationOfForeignEarnings	5400000	USD
CY2019	ati	Effective Income Tax Rate Reconciliation Restructuring EffectiveIncomeTaxRateReconciliationRestructuring	4200000	USD
CY2020Q4	us-gaap	Deferred Tax Assets Operating Loss Carryforwards DeferredTaxAssetsOperatingLossCarryforwards	309400000	USD
CY2019Q4	us-gaap	Deferred Tax Assets Operating Loss Carryforwards DeferredTaxAssetsOperatingLossCarryforwards	264400000	USD
CY2020Q4	us-gaap	Deferred Tax Assets Tax Deferred Expense Compensation And Benefits Pensions DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsPensions	153800000	USD
CY2019Q4	us-gaap	Deferred Tax Assets Tax Deferred Expense Compensation And Benefits Pensions DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsPensions	155500000	USD
CY2020Q4	us-gaap	Deferred Tax Assets Tax Deferred Expense Compensation And Benefits Postretirement Benefits DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsPostretirementBenefits	86500000	USD
CY2019Q4	us-gaap	Deferred Tax Assets Tax Deferred Expense Compensation And Benefits Postretirement Benefits DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsPostretirementBenefits	83800000	USD
CY2020Q4	us-gaap	Deferred Tax Assets Tax Credit Carryforwards DeferredTaxAssetsTaxCreditCarryforwards	37700000	USD
CY2019Q4	us-gaap	Deferred Tax Assets Tax Credit Carryforwards DeferredTaxAssetsTaxCreditCarryforwards	42600000	USD
CY2020Q4	us-gaap	Deferred Tax Assets Other DeferredTaxAssetsOther	82900000	USD
CY2019Q4	us-gaap	Deferred Tax Assets Other DeferredTaxAssetsOther	86100000	USD
CY2020Q4	us-gaap	Deferred Tax Assets Gross DeferredTaxAssetsGross	670300000	USD
CY2019Q4	us-gaap	Deferred Tax Assets Gross DeferredTaxAssetsGross	632400000	USD
CY2020Q4	us-gaap	Deferred Tax Assets Valuation Allowance DeferredTaxAssetsValuationAllowance	461800000	USD
CY2019Q4	us-gaap	Deferred Tax Assets Valuation Allowance DeferredTaxAssetsValuationAllowance	94500000	USD
CY2020Q4	us-gaap	Deferred Tax Assets Net DeferredTaxAssetsNet	208500000	USD
CY2019Q4	us-gaap	Deferred Tax Assets Net DeferredTaxAssetsNet	537900000	USD
CY2020Q4	us-gaap	Deferred Tax Liabilities Property Plant And Equipment DeferredTaxLiabilitiesPropertyPlantAndEquipment	119200000	USD
CY2019Q4	us-gaap	Deferred Tax Liabilities Property Plant And Equipment DeferredTaxLiabilitiesPropertyPlantAndEquipment	364200000	USD
CY2020Q4	ati	Deferred Tax Liabilities Inventory DeferredTaxLiabilitiesInventory	53300000	USD
CY2019Q4	ati	Deferred Tax Liabilities Inventory DeferredTaxLiabilitiesInventory	65500000	USD
CY2020Q4	us-gaap	Deferred Tax Liabilities Goodwill And Intangible Assets DeferredTaxLiabilitiesGoodwillAndIntangibleAssets	12500000	USD
CY2019Q4	us-gaap	Deferred Tax Liabilities Goodwill And Intangible Assets DeferredTaxLiabilitiesGoodwillAndIntangibleAssets	23700000	USD
CY2020Q4	us-gaap	Deferred Tax Liabilities Other DeferredTaxLiabilitiesOther	35900000	USD
CY2019Q4	us-gaap	Deferred Tax Liabilities Other DeferredTaxLiabilitiesOther	27000000.0	USD
CY2020Q4	us-gaap	Deferred Income Tax Liabilities DeferredIncomeTaxLiabilities	220900000	USD
CY2019Q4	us-gaap	Deferred Income Tax Liabilities DeferredIncomeTaxLiabilities	480400000	USD
CY2020Q4	us-gaap	Deferred Tax Liabilities DeferredTaxLiabilities	12400000	USD
CY2019Q4	us-gaap	Deferred Tax Assets Liabilities Net DeferredTaxAssetsLiabilitiesNet	57500000	USD
CY2020	us-gaap	Income Taxes Paid IncomeTaxesPaid	7800000	USD
CY2019	us-gaap	Income Taxes Paid IncomeTaxesPaid	15100000	USD
CY2018	us-gaap	Income Taxes Paid IncomeTaxesPaid	9700000	USD
CY2020	us-gaap	Proceeds From Income Tax Refunds ProceedsFromIncomeTaxRefunds	2500000	USD
CY2019	us-gaap	Proceeds From Income Tax Refunds ProceedsFromIncomeTaxRefunds	9200000	USD
CY2018	us-gaap	Proceeds From Income Tax Refunds ProceedsFromIncomeTaxRefunds	1600000	USD
CY2020	us-gaap	Income Taxes Paid Net IncomeTaxesPaidNet	5300000	USD
CY2019	us-gaap	Income Taxes Paid Net IncomeTaxesPaidNet	5900000	USD
CY2018	us-gaap	Income Taxes Paid Net IncomeTaxesPaidNet	8100000	USD
CY2020Q4	us-gaap	Deferred Tax Liabilities Undistributed Foreign Earnings DeferredTaxLiabilitiesUndistributedForeignEarnings	4400000	USD
CY2019Q4	us-gaap	Unrecognized Tax Benefits UnrecognizedTaxBenefits	14400000	USD
CY2018Q4	us-gaap	Unrecognized Tax Benefits UnrecognizedTaxBenefits	14700000	USD
CY2017Q4	us-gaap	Unrecognized Tax Benefits UnrecognizedTaxBenefits	14700000	USD
CY2020	us-gaap	Unrecognized Tax Benefits Increases Resulting From Prior Period Tax Positions UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions	0	USD
CY2019	us-gaap	Unrecognized Tax Benefits Increases Resulting From Prior Period Tax Positions UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions	0	USD
CY2018	us-gaap	Unrecognized Tax Benefits Increases Resulting From Prior Period Tax Positions UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions	0	USD
CY2020	us-gaap	Unrecognized Tax Benefits Decreases Resulting From Prior Period Tax Positions UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions	0	USD
CY2019	us-gaap	Unrecognized Tax Benefits Decreases Resulting From Prior Period Tax Positions UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions	0	USD
CY2018	us-gaap	Unrecognized Tax Benefits Decreases Resulting From Prior Period Tax Positions UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions	100000	USD
CY2020	us-gaap	Unrecognized Tax Benefits Increases Resulting From Current Period Tax Positions UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions	2700000	USD
CY2019	us-gaap	Unrecognized Tax Benefits Increases Resulting From Current Period Tax Positions UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions	900000	USD
CY2018	us-gaap	Unrecognized Tax Benefits Increases Resulting From Current Period Tax Positions UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions	700000	USD
CY2020	us-gaap	Unrecognized Tax Benefits Reductions Resulting From Lapse Of Applicable Statute Of Limitations UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations	1900000	USD
CY2019	us-gaap	Unrecognized Tax Benefits Reductions Resulting From Lapse Of Applicable Statute Of Limitations UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations	1200000	USD
CY2018	us-gaap	Unrecognized Tax Benefits Reductions Resulting From Lapse Of Applicable Statute Of Limitations UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations	600000	USD
CY2020	us-gaap	Unrecognized Tax Benefits Decreases Resulting From Settlements With Taxing Authorities UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities	0	USD
CY2019	us-gaap	Unrecognized Tax Benefits Decreases Resulting From Settlements With Taxing Authorities UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities	0	USD
CY2018	us-gaap	Unrecognized Tax Benefits Decreases Resulting From Settlements With Taxing Authorities UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities	0	USD
CY2020Q4	us-gaap	Unrecognized Tax Benefits UnrecognizedTaxBenefits	15200000	USD
CY2019Q4	us-gaap	Unrecognized Tax Benefits UnrecognizedTaxBenefits	14400000	USD
CY2018Q4	us-gaap	Unrecognized Tax Benefits UnrecognizedTaxBenefits	14700000	USD
CY2020Q4	ati	Unrecognized Tax Benefits Reductionof Operating Loss Carryforwards UnrecognizedTaxBenefitsReductionofOperatingLossCarryforwards	13000000.0	USD
CY2019Q4	ati	Unrecognized Tax Benefits Reductionof Operating Loss Carryforwards UnrecognizedTaxBenefitsReductionofOperatingLossCarryforwards	11500000	USD
CY2018Q4	ati	Unrecognized Tax Benefits Reductionof Operating Loss Carryforwards UnrecognizedTaxBenefitsReductionofOperatingLossCarryforwards	12100000	USD
CY2020Q4	us-gaap	Unrecognized Tax Benefits That Would Impact Effective Tax Rate UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate	2000000	USD
CY2019Q4	us-gaap	Unrecognized Tax Benefits Income Tax Penalties And Interest Accrued UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued	2700000	USD
CY2020Q4	us-gaap	Significant Change In Unrecognized Tax Benefits Is Reasonably Possible Amount Of Unrecorded Benefit SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleAmountOfUnrecordedBenefit	2000000	USD
CY2020Q4	us-gaap	Unrecognized Tax Benefits Income Tax Penalties And Interest Accrued UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued	2300000	USD
CY2020	us-gaap	Income Loss From Continuing Operations IncomeLossFromContinuingOperations	-1572600000	USD
CY2019	us-gaap	Income Loss From Continuing Operations IncomeLossFromContinuingOperations	257600000	USD
CY2018	us-gaap	Income Loss From Continuing Operations IncomeLossFromContinuingOperations	222400000	USD
CY2020	us-gaap	Net Income Loss Available To Common Stockholders Diluted NetIncomeLossAvailableToCommonStockholdersDiluted	-1572600000	USD
CY2019	us-gaap	Net Income Loss Available To Common Stockholders Diluted NetIncomeLossAvailableToCommonStockholdersDiluted	270400000	USD
CY2018	us-gaap	Net Income Loss Available To Common Stockholders Diluted NetIncomeLossAvailableToCommonStockholdersDiluted	235300000	USD
CY2020	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	126500000	shares
CY2019	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	125800000	shares
CY2018	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	125200000	shares
CY2020	us-gaap	Incremental Common Shares Attributable To Share Based Payment Arrangements IncrementalCommonSharesAttributableToShareBasedPaymentArrangements	0	shares
CY2019	us-gaap	Incremental Common Shares Attributable To Share Based Payment Arrangements IncrementalCommonSharesAttributableToShareBasedPaymentArrangements	800000	shares
CY2018	us-gaap	Incremental Common Shares Attributable To Share Based Payment Arrangements IncrementalCommonSharesAttributableToShareBasedPaymentArrangements	800000	shares
CY2020	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	126500000	shares
CY2019	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	146500000	shares
CY2018	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	145900000	shares
CY2020	us-gaap	Income Loss From Continuing Operations Per Basic Share IncomeLossFromContinuingOperationsPerBasicShare	-12.43
CY2019	us-gaap	Income Loss From Continuing Operations Per Basic Share IncomeLossFromContinuingOperationsPerBasicShare	2.05
CY2018	us-gaap	Income Loss From Continuing Operations Per Basic Share IncomeLossFromContinuingOperationsPerBasicShare	1.78
CY2020	us-gaap	Income Loss From Continuing Operations Per Diluted Share IncomeLossFromContinuingOperationsPerDilutedShare	-12.43
CY2019	us-gaap	Income Loss From Continuing Operations Per Diluted Share IncomeLossFromContinuingOperationsPerDilutedShare	1.85
CY2018	us-gaap	Income Loss From Continuing Operations Per Diluted Share IncomeLossFromContinuingOperationsPerDilutedShare	1.61
CY2020	us-gaap	Antidilutive Securities Excluded From Computation Of Earnings Per Share Amount AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount	22800000	shares
CY2018	us-gaap	Antidilutive Securities Excluded From Computation Of Earnings Per Share Amount AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount	0	shares
CY2019	us-gaap	Antidilutive Securities Excluded From Computation Of Earnings Per Share Amount AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount	0	shares
CY2020Q4	us-gaap	Unrecorded Unconditional Purchase Obligation Balance Sheet Amount UnrecordedUnconditionalPurchaseObligationBalanceSheetAmount	74900000	USD
CY2020Q4	us-gaap	Accrual For Environmental Loss Contingencies AccrualForEnvironmentalLossContingencies	14000000	USD
CY2020Q4	us-gaap	Accrued Environmental Loss Contingencies Current AccruedEnvironmentalLossContingenciesCurrent	4000000	USD
CY2020Q4	ati	Environmental Remediation Obligation For Formerly Owned Sites EnvironmentalRemediationObligationForFormerlyOwnedSites	9000000	USD
CY2020Q4	ati	Environmental Remediation Obligation For Owned Or Controlled Sites With Closed Operations EnvironmentalRemediationObligationForOwnedOrControlledSitesWithClosedOperations	1000000	USD
CY2020Q4	ati	Environmental Remediation Obligation Federal Superfund Sites EnvironmentalRemediationObligationFederalSuperfundSites	3000000	USD
CY2020Q4	ati	Environmental Remediation Obligation For Ongoing Operations EnvironmentalRemediationObligationForOngoingOperations	1000000	USD
CY2020Q4	us-gaap	Loss Contingency Range Of Possible Loss Portion Not Accrued LossContingencyRangeOfPossibleLossPortionNotAccrued	15000000	USD
CY2020Q1	us-gaap	Revenue From Contract With Customer Excluding Assessed Tax RevenueFromContractWithCustomerExcludingAssessedTax	955500000	USD
CY2020Q2	us-gaap	Revenue From Contract With Customer Excluding Assessed Tax RevenueFromContractWithCustomerExcludingAssessedTax	770300000	USD
CY2020Q3	us-gaap	Revenue From Contract With Customer Excluding Assessed Tax RevenueFromContractWithCustomerExcludingAssessedTax	598000000.0	USD
CY2020Q4	us-gaap	Revenue From Contract With Customer Excluding Assessed Tax RevenueFromContractWithCustomerExcludingAssessedTax	658300000	USD
CY2020Q1	us-gaap	Gross Profit GrossProfit	134800000	USD
CY2020Q2	us-gaap	Gross Profit GrossProfit	74700000	USD
CY2020Q3	us-gaap	Gross Profit GrossProfit	38100000	USD
CY2020Q4	us-gaap	Gross Profit GrossProfit	45200000	USD
CY2020Q1	us-gaap	Profit Loss ProfitLoss	23600000	USD
CY2020Q2	us-gaap	Profit Loss ProfitLoss	-419900000	USD
CY2020Q4	us-gaap	Profit Loss ProfitLoss	-1116300000	USD
CY2020Q1	us-gaap	Net Income Loss NetIncomeLoss	21100000	USD
CY2020Q3	us-gaap	Profit Loss ProfitLoss	-47000000.0	USD
CY2020Q2	us-gaap	Net Income Loss NetIncomeLoss	-422600000	USD
CY2020Q3	us-gaap	Net Income Loss NetIncomeLoss	-50100000	USD
CY2020Q4	us-gaap	Net Income Loss NetIncomeLoss	-1121000000.0	USD
CY2020Q1	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	0.17
CY2020Q2	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	-3.34
CY2020Q3	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	-0.40
CY2020Q4	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	-8.85
CY2020Q1	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	0.16
CY2020Q2	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	-3.34
CY2020Q3	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	-0.40
CY2020Q4	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	-8.85
CY2020Q1	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	126300000	shares
CY2020Q2	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	126700000	shares
CY2020Q3	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	126800000	shares
CY2020Q4	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	126800000	shares
CY2019Q1	us-gaap	Revenue From Contract With Customer Excluding Assessed Tax RevenueFromContractWithCustomerExcludingAssessedTax	1004800000	USD
CY2019Q2	us-gaap	Revenue From Contract With Customer Excluding Assessed Tax RevenueFromContractWithCustomerExcludingAssessedTax	1080400000	USD
CY2019Q3	us-gaap	Revenue From Contract With Customer Excluding Assessed Tax RevenueFromContractWithCustomerExcludingAssessedTax	1018700000	USD
CY2019Q4	us-gaap	Revenue From Contract With Customer Excluding Assessed Tax RevenueFromContractWithCustomerExcludingAssessedTax	1018600000	USD
CY2019Q1	us-gaap	Gross Profit GrossProfit	131100000	USD
CY2019Q2	us-gaap	Gross Profit GrossProfit	177700000	USD
CY2019Q3	us-gaap	Gross Profit GrossProfit	159700000	USD
CY2019Q4	us-gaap	Gross Profit GrossProfit	169300000	USD
CY2019Q1	us-gaap	Profit Loss ProfitLoss	16300000	USD
CY2019Q2	us-gaap	Profit Loss ProfitLoss	78500000	USD
CY2019Q3	us-gaap	Profit Loss ProfitLoss	115300000	USD
CY2019Q4	us-gaap	Profit Loss ProfitLoss	60000000.0	USD
CY2019Q1	us-gaap	Net Income Loss NetIncomeLoss	15000000.0	USD
CY2019Q2	us-gaap	Net Income Loss NetIncomeLoss	75100000	USD
CY2019Q3	us-gaap	Net Income Loss NetIncomeLoss	111000000.0	USD
CY2019Q4	ati	Restructuring And Other Charges Net Of Tax RestructuringAndOtherChargesNetOfTax	4300000	USD
CY2019Q4	us-gaap	Net Income Loss NetIncomeLoss	56500000	USD
CY2019Q1	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	0.12
CY2019Q2	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	0.60
CY2019Q3	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	0.88
CY2019Q4	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	0.45
CY2019Q1	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	0.12
CY2019Q2	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	0.54
CY2019Q3	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	0.78
CY2019Q4	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	0.41
CY2019Q1	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	125800000	shares
CY2019Q2	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	126100000	shares
CY2019Q3	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	126100000	shares
CY2019Q4	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	126100000	shares
CY2020Q1	ati	Restructuringandothercharges Restructuringandothercharges	8000000.0	USD
CY2020Q1	ati	Restructuring And Other Charges Net Of Tax RestructuringAndOtherChargesNetOfTax	5500000	USD
CY2020Q2	us-gaap	Goodwill Impairment Loss GoodwillImpairmentLoss	287000000.0	USD
CY2020Q2	us-gaap	Goodwill Impairment Loss Net Of Tax GoodwillImpairmentLossNetOfTax	281400000	USD
CY2020Q2	ati	Restructuringandothercharges Restructuringandothercharges	16700000	USD
CY2020Q2	ati	Restructuring And Other Charges Net Of Tax RestructuringAndOtherChargesNetOfTax	16400000	USD
CY2020Q2	ati	Other Asset Impairment Charges And Due From Affiliates Allowance For Credit Loss OtherAssetImpairmentChargesAndDueFromAffiliatesAllowanceForCreditLoss	2400000	USD
CY2020Q2	us-gaap	Gains Losses On Extinguishment Of Debt GainsLossesOnExtinguishmentOfDebt	-21500000	USD
CY2020Q2	us-gaap	Extinguishment Of Debt Gain Loss Net Of Tax ExtinguishmentOfDebtGainLossNetOfTax	-21100000	USD
CY2020Q2	us-gaap	Valuation Allowance Deferred Tax Asset Change In Amount ValuationAllowanceDeferredTaxAssetChangeInAmount	99000000.0	USD
CY2020Q3	ati	Restructuringandothercharges Restructuringandothercharges	2300000	USD
CY2020Q4	ati	Restructuringandothercharges Restructuringandothercharges	1105100000	USD
CY2020Q4	us-gaap	Other Asset Impairment Charges OtherAssetImpairmentCharges	1041500000	USD
CY2020Q4	us-gaap	Severance Costs1 SeveranceCosts1	33500000	USD
CY2020Q4	us-gaap	Other Restructuring Costs OtherRestructuringCosts	12700000	USD
CY2020Q4	us-gaap	Defined Benefit Plan Cost Of Providing Special Or Contractual Termination Benefit Recognized During Period DefinedBenefitPlanCostOfProvidingSpecialOrContractualTerminationBenefitRecognizedDuringPeriod	17400000	USD
CY2020Q4	us-gaap	Valuation Allowance Deferred Tax Asset Change In Amount ValuationAllowanceDeferredTaxAssetChangeInAmount	-26000000.0	USD
CY2019Q2	us-gaap	Gain Loss On Sale Of Property GainLossOnSaleOfProperty	29300000	USD
CY2019Q2	ati	Gain Loss On Disposition Of Oil And Gas And Timber Property Net Of Tax GainLossOnDispositionOfOilAndGasAndTimberPropertyNetOfTax	27300000	USD
CY2019Q3	us-gaap	Gain Loss On Sale Of Property GainLossOnSaleOfProperty	62400000	USD
CY2019Q3	ati	Gain Loss On Disposition Of Oil And Gas And Timber Property Net Of Tax GainLossOnDispositionOfOilAndGasAndTimberPropertyNetOfTax	60500000	USD
CY2019Q4	ati	Restructuringandothercharges Restructuringandothercharges	4500000	USD
CY2019Q4	us-gaap	Gains Losses On Extinguishment Of Debt GainsLossesOnExtinguishmentOfDebt	-21600000	USD
CY2019Q4	us-gaap	Extinguishment Of Debt Gain Loss Net Of Tax ExtinguishmentOfDebtGainLossNetOfTax	-20500000	USD
CY2019Q4	ati	Other Asset Impairment Charges And Due From Affiliates Allowance For Credit Loss OtherAssetImpairmentChargesAndDueFromAffiliatesAllowanceForCreditLoss	11400000	USD
CY2019Q4	ati	Other Asset Impairment Charges And Due From Affiliates Allowance For Credit Loss Net Of Tax OtherAssetImpairmentChargesAndDueFromAffiliatesAllowanceForCreditLossNetOfTax	10800000	USD
CY2019Q4	us-gaap	Valuation Allowance Deferred Tax Asset Change In Amount ValuationAllowanceDeferredTaxAssetChangeInAmount	-41900000	USD

Files In Submission

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$ATI
ATI INC Stock $ati

2020 Q4 Form 10-K Financial Statement

#000162828021003434 Filed on February 26, 2021

Income Statement

Balance Sheet

Cashflow Statement

Facts In Submission

Files In Submission

$ATI ATI INC Stock $ati

2020 Q4 Form 10-K Financial Statement

#000162828021003434 Filed on February 26, 2021

Income Statement

Balance Sheet

Cashflow Statement

Facts In Submission

Files In Submission

$ATI
ATI INC Stock $ati