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Debt - Schedule of Debt (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2016
Feb. 15, 2016
Dec. 31, 2013
Debt Instrument [Line Items]          
Debt Issuance Costs, Net $ 12.3 $ 10.4      
Total short-term and long-term debt 1,398.9 1,542.1      
Short-term debt and current portion of long-term debt 11.5 6.6      
Total long-term debt $ 1,387.4 $ 1,535.5      
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       7.875% 5.875%
Debt instrument carrying amount $ 500.0 $ 500.0      
Allegheny Technologies $500 million 5.95% Senior Notes due 2021 [Member]          
Debt Instrument [Line Items]          
Debt instrument issuer   Allegheny Technologies      
Debt instrument interest rate stated percentage   5.95%      
Debt instrument carrying amount $ 0.0 $ 500.0      
Allegheny Technologies $350 million 5.875% Senior Notes due 2027          
Debt Instrument [Line Items]          
Debt instrument issuer Allegheny Technologies        
Debt instrument interest rate stated percentage 5.875%        
Debt instrument carrying amount $ 350.0 $ 0.0      
Allegheny Technologies $350 million 9.375% Senior Notes due 2019          
Debt Instrument [Line Items]          
Debt instrument interest rate stated percentage     9.375%    
Debt instrument carrying amount     $ 350.0    
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 $ 287.5 $ 287.5      
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 $ 150.0 $ 150.0      
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 $ 100.0 $ 100.0      
Domestic Bank Group $500 million asset-based credit facility [Member]          
Debt Instrument [Line Items]          
Debt instrument carrying amount 0.0 0.0      
Finance leases and other [Member]          
Debt Instrument [Line Items]          
Debt instrument carrying amount 18.8 15.0      
Foreign credit agreements [Member]          
Debt Instrument [Line Items]          
Debt instrument carrying amount $ 4.9 $ 0.0      
Maximum | Allegheny Technologies $500 million 5.875% Senior Notes due 2023 [Member]          
Debt Instrument [Line Items]          
Debt instrument interest rate stated percentage 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.