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Debt Schedule of Debt (Details) - USD ($)
$ in Millions
6 Months Ended 12 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Feb. 15, 2016
Dec. 31, 2013
Debt Instrument [Line Items]        
Debt Issuance Costs, Net $ 11.8 $ 13.1    
Total Debt 1,551.8 1,540.7    
Short-term debt and current portion of long-term debt 16.3 10.1    
Long-term debt 1,535.5 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    
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 9.7 6.3    
Other        
Debt Instrument [Line Items]        
Long-term Debt, Gross $ 16.4 $ 10.0    
[1] Bearing interest at 7.875% effective February 15, 2016.