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Debt Narrative (Details)
1 Months Ended 9 Months Ended
Jul. 31, 2018
USD ($)
Sep. 30, 2018
USD ($)
Sep. 30, 2017
USD ($)
Dec. 31, 2017
USD ($)
Feb. 15, 2016
Dec. 31, 2013
Allegheny Technologies 5.875% Notes due 2023 (a)            
Debt Instrument [Line Items]            
Interest rate   7.875%   7.875% 7.875% 5.875%
Outstanding borrowings [1]   $ 500,000,000   $ 500,000,000    
Asset-based lending credit facility            
Debt Instrument [Line Items]            
Maximum borrowing capacity   500,000,000        
Domestic Bank Group $400 million asset-based credit facility            
Debt Instrument [Line Items]            
Short-term Debt   0   0    
Amount utilized to support the issuance of letters of credit   35,100,000        
Average borrowings during period   $ 56,000,000 $ 46,000,000      
Interest rate during period   3.651% 3.276%      
Domestic Bank Group $400 million asset-based credit facility | Revolving credit facility            
Debt Instrument [Line Items]            
Maximum borrowing capacity   $ 400,000,000        
Minimum fixed charge coverage ratio allowed in event of default   1        
Percentage of borrowing capacity unavailable due to fixed charge coverage ratio covenant   10.00%        
Borrowing capacity unavailable due to fixed charge coverage ratio covenant   $ 40,000,000        
Minimum required liquidity number of days prior to maturity of Senior Notes   91 days        
Domestic Bank Group $400 million asset-based credit facility | Revolving credit facility | Minimum | LIBOR based borrowings            
Debt Instrument [Line Items]            
Variable interest rate (percentage)   1.75%        
Domestic Bank Group $400 million asset-based credit facility | Revolving credit facility | Minimum | Base rate borrowings            
Debt Instrument [Line Items]            
Variable interest rate (percentage)   1.00%        
Domestic Bank Group $400 million asset-based credit facility | Revolving credit facility | Maximum | LIBOR based borrowings            
Debt Instrument [Line Items]            
Variable interest rate (percentage)   2.25%        
Domestic Bank Group $400 million asset-based credit facility | Revolving credit facility | Maximum | Base rate borrowings            
Debt Instrument [Line Items]            
Variable interest rate (percentage)   1.50%        
Domestic Bank Group $400 million asset-based credit facility | Letter of credit sub-facility            
Debt Instrument [Line Items]            
Maximum borrowing capacity   $ 200,000,000        
2022 Term Loan            
Debt Instrument [Line Items]            
Interest rate 5.44%          
Minimum prepayment increments allowed   50,000,000        
Outstanding borrowings   $ 100,000,000   $ 100,000,000    
2022 Term Loan | Interest Rate Swap            
Debt Instrument [Line Items]            
Notional amount of derivative $ 50,000,000          
2022 Term Loan | LIBOR based borrowings            
Debt Instrument [Line Items]            
Variable interest rate (percentage) 2.50% 3.00%        
Allegheny Technologies, Convertible Senior Notes, 4.75%, Due 2022            
Debt Instrument [Line Items]            
Interest rate   4.75%   4.75%    
Outstanding borrowings   $ 287,500,000   $ 287,500,000    
Allegheny Technologies 5.95% Notes due 2021            
Debt Instrument [Line Items]            
Interest rate   5.95%   5.95%    
Outstanding borrowings   $ 500,000,000   $ 500,000,000    
Allegheny Technologies 5.95% Notes due 2021 | Revolving credit facility            
Debt Instrument [Line Items]            
Minimum required liquidity prior to maturity of Senior Notes   $ 700,000,000        
[1] Bearing interest at 7.875% effective February 15, 2016.