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Debt - Narrative (Details)
 3 Months Ended 12 Months Ended
Jun. 02, 2016
USD ($)
$ / shares
Dec. 31, 2017
USD ($)
Sep. 30, 2017
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
May 24, 2016
USD ($)
Feb. 15, 2016
 [1]
Dec. 31, 2014
Dec. 31, 2013
Interest Costs Incurred [Abstract]                    
Interest expense       $ 134,900,000 $ 125,400,000 $ 111,400,000        
Interest costs capitalized       2,600,000 4,700,000 2,200,000        
Interest costs paid       133,800,000 127,200,000 113,400,000        
Interest income       1,100,000 1,400,000 1,200,000        
Long-term Debt, Fiscal Year Maturity [Abstract]                    
2018   $ 10,100,000   10,100,000            
2019   4,500,000   4,500,000            
2020   1,100,000   1,100,000            
2021   500,500,000   500,500,000            
2022   387,700,000.0   387,700,000.0            
Debt Instrument [Line Items]                    
Payments of debt issuance costs       800,000 10,400,000 0        
Debt extinguishment charge   37,000,000   37,000,000 0 0        
Payment for Debt Extinguishment or Debt Prepayment Cost       35,800,000 0 0        
Write off of Deferred Debt Issuance Cost       $ 1,200,000            
Allegheny Technologies $500 million 5.875% Senior Notes due 2023 [Member]                    
Debt Instrument [Line Items]                    
Debt instrument interest rate stated percentage               7.875%   5.875%
Increase in interest expense per downgrade notch, percent       0.25%            
Maximum number of downgrade notches       4            
Maximum increase in interest rate expense, percent       2.00%            
Additional annual interest expense to be incurred         1,300,000          
Debt instrument carrying amount [1]   $ 500,000,000   $ 500,000,000 500,000,000          
Allegheny Technologies $500 million 5.875% Senior Notes due 2023 [Member] | Maximum [Member]                    
Debt Instrument [Line Items]                    
Debt instrument interest rate stated percentage   7.875%   7.875%            
Asset-based lending credit facility [Member]                    
Debt Instrument [Line Items]                    
Line of credit maximum borrowing capacity   $ 500,000,000   $ 500,000,000            
Domestic Bank Group $400 million credit agreement [Member]                    
Debt Instrument [Line Items]                    
Average outstanding amount       $ 37,000,000 $ 82,000,000          
Interest rate during period       3.255% 1.80%          
Domestic Bank Group $400 million asset-based credit facility [Member]                    
Debt Instrument [Line Items]                    
Debt instrument carrying amount   0   $ 0 $ 0          
Line of credit facility amount outstanding   42,300,000   42,300,000            
2022 Term Loan [Member]                    
Debt Instrument [Line Items]                    
Payments of debt issuance costs       800,000 1,000,000          
Debt instrument carrying amount   $ 100,000,000   100,000,000 $ 100,000,000          
Debt Instrument, Prepayment Increments, Minimum       $ 50,000,000            
2022 Term Loan [Member] | London Interbank Offered Rate (LIBOR) [Member]                    
Debt Instrument [Line Items]                    
Variable rate spread     3.00%   3.50%          
Allegheny Technologies, Convertible Senior Notes, 4.75%, Due 2022 [Member]                    
Debt Instrument [Line Items]                    
Debt instrument interest rate stated percentage   4.75%   4.75% 4.75%          
Debt instrument carrying amount   $ 287,500,000   $ 287,500,000 $ 287,500,000          
Allegheny Technologies, Convertible Senior Notes, 4.75%, Due 2022 [Member] | Convertible Debt [Member]                    
Debt Instrument [Line Items]                    
Debt instrument interest rate stated percentage   4.75%   4.75%            
Payments of debt issuance costs $ 9,400,000                  
Amortization period of debt issuance costs 6 years                  
Long-term debt             $ 287,500,000      
Convertible debt conversion rate in shares of stock 0.0692042                  
Number of equity instruments issuable 19,900,000                  
Conversion price of convertible notes | $ / shares $ 14.45                  
Allegheny Technologies $350 million 9.375% Senior Notes due 2019 [Member]                    
Debt Instrument [Line Items]                    
Debt instrument interest rate stated percentage         9.375%          
Debt instrument carrying amount   $ 0   $ 0 $ 350,000,000          
Ladish Series B 6.14% Notes due 2016 [Member]                    
Debt Instrument [Line Items]                    
Debt instrument interest rate stated percentage                 6.14%  
Repayments of debt           5,700,000        
Ladish Series C 6.41% Notes due 2015 [Member]                    
Debt Instrument [Line Items]                    
Debt instrument interest rate stated percentage                 6.41%  
Repayments of debt           $ 10,000,000        
STAL Revolving Credit Facility [Member]                    
Debt Instrument [Line Items]                    
Line of credit maximum borrowing capacity   $ 20,000,000   $ 20,000,000            
Joint venture ownership percentage   60.00%   60.00%            
Revolving Credit Facility [Member] | Domestic Bank Group $400 million asset-based credit facility [Member]                    
Debt Instrument [Line Items]                    
Line of credit maximum borrowing capacity   $ 400,000,000   $ 400,000,000            
Payments of debt issuance costs       $ 1,000,000 $ 2,400,000          
Fixed charge coverage ratio   1   1            
Remaining borrowing capacity, percent of minimum borrowing capacity   10.00%   10.00%            
Line of Credit Facility Borrowing Capacity Unavailable Due to Covenants   $ 50,000,000   $ 50,000,000            
Minimum remaining borrowing capacity   40,000,000   40,000,000            
Minimum required liquidity   700,000,000   $ 700,000,000            
Minimum required liquidity, number of days prior       91 days            
Revolving Credit Facility [Member] | Domestic Bank Group $400 million asset-based credit facility [Member] | Minimum [Member] | London Interbank Offered Rate (LIBOR) [Member]                    
Debt Instrument [Line Items]                    
Variable rate spread       1.75% 2.00%          
Revolving Credit Facility [Member] | Domestic Bank Group $400 million asset-based credit facility [Member] | Minimum [Member] | Base Rate [Member]                    
Debt Instrument [Line Items]                    
Variable rate spread       1.00%            
Revolving Credit Facility [Member] | Domestic Bank Group $400 million asset-based credit facility [Member] | Maximum [Member] | London Interbank Offered Rate (LIBOR) [Member]                    
Debt Instrument [Line Items]                    
Variable rate spread       2.25% 2.50%          
Revolving Credit Facility [Member] | Domestic Bank Group $400 million asset-based credit facility [Member] | Maximum [Member] | Base Rate [Member]                    
Debt Instrument [Line Items]                    
Variable rate spread       1.50%            
Letter of Credit [Member] | Domestic Bank Group $400 million asset-based credit facility [Member]                    
Debt Instrument [Line Items]                    
Line of credit maximum borrowing capacity   $ 200,000,000   $ 200,000,000            
[1] Bearing interest at 7.875% effective February 15, 2016.