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Debt Schedule of Debt (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2020
Dec. 31, 2019
Feb. 15, 2016
Dec. 31, 2013
Debt Instrument [Line Items]        
Debt Issuance Costs, Net $ (11.7) $ (12.3)    
Total Debt 1,700.7 1,398.9    
Short-term debt and current portion of long-term debt 310.7 11.5    
Long-term debt 1,390.0 1,387.4    
Allegheny Technologies, Convertible Senior Notes, 4.75%, Due 2022        
Debt Instrument [Line Items]        
Debt, Gross $ 287.5 $ 287.5    
Debt Instrument, Issuer Allegheny Technologies      
Debt Instrument, Maturity Date Jul. 01, 2022 Jul. 01, 2022    
Interest rate 4.75% 4.75%    
Allegheny Technologies 5.875% Notes due 2023 (a)        
Debt Instrument [Line Items]        
Debt, Gross [1] $ 500.0 $ 500.0    
Debt Instrument, Issuer Allegheny Technologies      
Debt Instrument, Maturity Date Aug. 15, 2023 Aug. 15, 2023    
Interest rate 7.875% 7.875% 7.875% 5.875%
ATI 2027 Notes [Member]        
Debt Instrument [Line Items]        
Debt, Gross $ 350.0 $ 350.0    
Debt Instrument, Issuer Allegheny Technologies      
Debt Instrument, Maturity Date Dec. 01, 2027 Dec. 01, 2027    
Interest rate 5.875% 5.875%    
Allegheny Ludlum 6.95% debentures due 2025        
Debt Instrument [Line Items]        
Debt, Gross $ 150.0 $ 150.0    
Debt Instrument, Issuer Allegheny Ludlum      
Debt Instrument, Maturity Date Dec. 15, 2025 Dec. 15, 2025    
Interest rate 6.95% 6.95%    
2024 Term Loan        
Debt Instrument [Line Items]        
Debt, Gross $ 100.0 $ 100.0    
Debt Instrument, Issuer Allegheny Technologies      
Debt Instrument, Maturity Date Sep. 30, 2024 Sep. 30, 2024    
Interest rate 4.21%      
Domestic Bank Group $500 million asset-based credit facility        
Debt Instrument [Line Items]        
Short-term Debt $ 300.0 $ 0.0    
Foreign credit facilities        
Debt Instrument [Line Items]        
Short-term Debt 3.5 4.9    
Other        
Debt Instrument [Line Items]        
Debt, Gross $ 21.4 $ 18.8    
[1] Bearing interest at 7.875% effective February 15, 2016.