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Debt - Schedule of Debt (Details) - USD ($)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Jul. 02, 2020
Jun. 30, 2020
Dec. 31, 2013
Debt Instrument [Line Items]          
Debt Issuance Costs, Net $ 14,500,000 $ 12,300,000      
Total short-term and long-term debt 1,567,800,000 1,398,900,000      
Short-term debt and current portion of long-term debt 17,800,000 11,500,000      
Total long-term debt $ 1,550,000,000.0 $ 1,387,400,000      
Allegheny Technologies $500 million 5.875% Senior Notes due 2023 [Member]          
Debt Instrument [Line Items]          
Debt instrument issuer [1] Allegheny Technologies Allegheny Technologies      
Debt instrument interest rate stated percentage         5.875%
Debt instrument carrying amount $ 500,000,000.0 $ 500,000,000.0      
Allegheny Technologies $350 million 5.875% Senior Notes due 2027          
Debt Instrument [Line Items]          
Debt instrument issuer Allegheny Technologies Allegheny Technologies      
Debt instrument interest rate stated percentage 5.875% 5.875%      
Debt instrument carrying amount $ 350,000,000.0 $ 350,000,000.0      
Debt Instrument, Maturity Date Dec. 01, 2027 Dec. 01, 2027      
Allegheny Technologies, $287.5 million Convertible Senior Notes, 4.75%, Due 2022 [Member]          
Debt Instrument [Line Items]          
Debt instrument issuer Allegheny Technologies Allegheny Technologies      
Debt instrument interest rate stated percentage 4.75% 4.75%      
Debt instrument carrying amount $ 84,200,000 $ 287,500,000      
Debt Instrument, Maturity Date Jul. 01, 2022 Jul. 01, 2022      
Allegheny Ludlum 6.95% Debentures due 2025 [Member]          
Debt Instrument [Line Items]          
Debt instrument issuer Allegheny Ludlum Allegheny Ludlum      
Debt instrument interest rate stated percentage 6.95% 6.95%      
Debt instrument carrying amount $ 150,000,000.0 $ 150,000,000.0      
Debt Instrument, Maturity Date Dec. 15, 2025 Dec. 15, 2025      
Term Loan due 2024          
Debt Instrument [Line Items]          
Debt instrument issuer Allegheny Technologies Allegheny Technologies      
Debt instrument interest rate stated percentage 4.21%        
Debt instrument carrying amount $ 200,000,000.0 $ 100,000,000.0      
Debt Instrument, Maturity Date Sep. 30, 2024 Sep. 30, 2024      
Domestic Bank Group $500 million asset-based credit facility [Member]          
Debt Instrument [Line Items]          
Debt instrument carrying amount $ 0 $ 0      
Allegheny Technologies, Senior Notes, 5.875, Due 2023 [Member]          
Debt Instrument [Line Items]          
Debt Instrument, Maturity Date Aug. 15, 2023 Aug. 15, 2023      
Finance leases and other [Member]          
Debt Instrument [Line Items]          
Debt instrument carrying amount $ 48,000,000.0 $ 18,800,000      
Foreign credit agreements [Member]          
Debt Instrument [Line Items]          
Debt instrument carrying amount $ 5,500,000 4,900,000      
Convertible Debt [Member] | Allegheny Technologies, $287.5 million Convertible Senior Notes, 4.75%, Due 2022 [Member]          
Debt Instrument [Line Items]          
Debt instrument interest rate stated percentage 4.75%        
Convertible Debt [Member] | Allegheny Technologies, Convertible Senior Notes, 3.5%, Due 2025 [Member]          
Debt Instrument [Line Items]          
Debt instrument issuer Allegheny Technologies        
Debt instrument interest rate stated percentage 3.50%        
Debt instrument carrying amount $ 291,400,000 0 $ 6,400,000 $ 285,000,000.0  
Equity component of convertible debt $ (46,800,000) $ 0 $ (51,400,000)    
Debt Instrument, Maturity Date Jun. 15, 2025        
Maximum | Allegheny Technologies $500 million 5.875% Senior Notes due 2023 [Member]          
Debt Instrument [Line Items]          
Debt instrument interest rate stated percentage 7.875% 7.875%      
London Interbank Offered Rate (LIBOR) [Member] | Term Loan due 2024          
Debt Instrument [Line Items]          
Variable rate spread 2.00%        
Revolving Credit Facility [Member] | London Interbank Offered Rate (LIBOR) [Member] | Minimum | Domestic Bank Group $500 million asset-based credit facility [Member]          
Debt Instrument [Line Items]          
Variable rate spread 1.25%        
Revolving Credit Facility [Member] | London Interbank Offered Rate (LIBOR) [Member] | Maximum | Domestic Bank Group $500 million asset-based credit facility [Member]          
Debt Instrument [Line Items]          
Variable rate spread 1.75%        
[1] Bearing interest at 7.875% effective February 15, 2016.(b)The payment obligations of these debentures issued by Allegheny Ludlum, LLC are fully and unconditionally guaranteed by ATI.