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Debt Schedule of Debt (Details) - USD ($)
$ in Millions
9 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Jul. 31, 2018
Feb. 15, 2016
Dec. 31, 2013
Debt Instrument [Line Items]          
Debt Issuance Costs, Net $ 11.1 $ 13.1      
Total Debt 1,551.9 1,540.7      
Short-term debt and current portion of long-term debt 16.6 10.1      
Long-term debt 1,535.3 1,530.6      
Allegheny Technologies, Convertible Senior Notes, 4.75%, Due 2022          
Debt Instrument [Line Items]          
Long-term 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]          
Long-term 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%
Allegheny Technologies 5.95% Notes due 2021          
Debt Instrument [Line Items]          
Long-term Debt, Gross $ 500.0 $ 500.0      
Debt Instrument, Issuer Allegheny Technologies        
Debt Instrument, Maturity Date Jan. 15, 2021 Jan. 15, 2021      
Interest rate 5.95% 5.95%      
Allegheny Ludlum 6.95% debentures due 2025          
Debt Instrument [Line Items]          
Long-term 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%      
2022 Term Loan          
Debt Instrument [Line Items]          
Long-term Debt, Gross $ 100.0 $ 100.0      
Debt Instrument, Issuer Allegheny Technologies        
Debt Instrument, Maturity Date Feb. 28, 2022 Feb. 28, 2022      
Interest rate     5.44%    
Domestic Bank Group $400 million asset-based credit facility          
Debt Instrument [Line Items]          
Short-term Debt $ 0.0 $ 0.0      
Foreign credit facilities          
Debt Instrument [Line Items]          
Short-term Debt 10.0 6.3      
Other          
Debt Instrument [Line Items]          
Long-term Debt, Gross $ 15.5 $ 10.0      
[1] Bearing interest at 7.875% effective February 15, 2016.