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Business Segments - Additional Information (Details)
3 Months Ended 6 Months Ended
Jun. 30, 2021
USD ($)
Jun. 30, 2020
USD ($)
Jun. 30, 2021
USD ($)
segment
Jun. 30, 2020
USD ($)
Dec. 31, 2020
USD ($)
Restructuring Cost and Reserve [Line Items]          
Number of business segments | segment     2    
Strike related costs $ 40,300,000 $ 0 $ 40,300,000 $ 0  
Impairment of goodwill 0 (287,000,000.0) 0 (287,000,000.0)  
Debt extinguishment charge (See Note 7) 0 $ (21,500,000) 0 (21,500,000)  
Gain on the sale of oil and gas rights       $ 2,500,000  
Allegheny Technologies 3.5% Convertible Senior Notes due 2025 | Convertible Debt          
Restructuring Cost and Reserve [Line Items]          
Debt, Gross 291,400,000   $ 291,400,000   $ 291,400,000
High Performance Materials & Components | Operating Segments          
Restructuring Cost and Reserve [Line Items]          
Strike related costs 2,100,000        
Advanced Alloys & Solutions [Member] | Operating Segments          
Restructuring Cost and Reserve [Line Items]          
Strike related costs $ 38,200,000