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Selected Quarterly Financial Data (Unaudited) - Narrative (Details) - USD ($)
shares in Millions, $ in Millions
 3 Months Ended 12 Months Ended
Nov. 07, 2017
Dec. 31, 2017
Sep. 30, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2015
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Unusual Or Infrequent Item [Line Items]                      
Restructuring costs                   $ 48.8  
Income taxes (above) below normal       $ (32.4)   $ (11.4) $ 12.0        
Federal tax rate                   35.00%  
NRV charge                 $ (54.0) $ (39.9) $ 131.5
Deferred tax asset valuation allowance charge       (22.5) $ 173.1            
Impairment of goodwill     $ 114.4           114.4 0.0 126.6
Impairment of goodwill net of tax     $ 113.6                
Debt extinguishment charge   $ 37.0             37.0 0.0 0.0
Change in federal tax rate and law change   (4.1)             (4.1) 0.0 0.0
Shares issued 17                    
Flat Rolled Products [Member]                      
Unusual Or Infrequent Item [Line Items]                      
Impairment of goodwill               $ 126.6      
High Performance Materials & Components and Corporate [Member]                      
Unusual Or Infrequent Item [Line Items]                      
Severance and termination benefit costs               6.3      
Employee Severance [Member] | Flat Rolled Products [Member]                      
Unusual Or Infrequent Item [Line Items]                      
Restructuring costs             9.0        
Restructuring costs net of tax             6.7        
Work Stoppage and Return to Work [Member] | Flat Rolled Products [Member]                      
Unusual Or Infrequent Item [Line Items]                      
Restructuring costs           22.4 26.4        
Restructuring costs net of tax           $ 8.4 $ 19.7        
Long-lived asset impairment [Member]                      
Unusual Or Infrequent Item [Line Items]                      
Restructuring costs         471.3            
Restructuring costs net of tax         310.3            
Asset Impairment Charges                     $ 54.5
Facility Closing [Member]                      
Unusual Or Infrequent Item [Line Items]                      
Restructuring costs       15.4 16.3         28.0  
Restructuring costs net of tax       10.0 10.7            
Employee benefit costs [Member]                      
Unusual Or Infrequent Item [Line Items]                      
Restructuring costs       13.2           20.8  
Restructuring costs net of tax       8.6              
Inventory Valuation Reserve [Member] | Titanium sponge inventory [Member]                      
Unusual Or Infrequent Item [Line Items]                      
Inventory valuation charges         11.3            
Inventory valuation charges net of tax         $ 7.4            
Inventory Valuation Reserve [Member] | Non-PQ grade titanium sponge [Member]                      
Unusual Or Infrequent Item [Line Items]                      
Inventory valuation charges               $ 25.4      
Allegheny Technologies $350 million 9.375% Senior Notes due 2019 [Member]                      
Unusual Or Infrequent Item [Line Items]                      
Debt instrument carrying amount   $ 0.0   $ 350.0         $ 0.0 $ 350.0  
Debt Instrument, Interest Rate, Stated Percentage       9.375%           9.375%