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Fair Value of Financial Instruments (Details) - USD ($)
$ in Millions
Dec. 31, 2021
Dec. 31, 2020
Jul. 02, 2020
Allegheny Technologies, Convertible Senior Notes, 3.5%, Due 2025 [Member] | Convertible Debt [Member]      
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]      
Equity component of convertible debt $ 0.0 $ 46.8 $ 51.4
Fair Value Measurements at Reporting Date Using Quoted Prices in Active Markets for Identical Assets (Level 1)      
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]      
Cash and cash equivalents 687.7 645.9  
Derivative financial instruments: Assets 0.0 0.0  
Derivative financial instruments: Liabilities 0.0 0.0  
Debt 1,690.1 1,594.2  
Fair Value Measurements at Reporting Date Using Significant Observable Inputs (Level 2)      
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]      
Cash and cash equivalents 0.0 0.0  
Derivative financial instruments: Assets 7.6 4.8  
Derivative financial instruments: Liabilities 2.7 4.0  
Debt 313.1 253.5  
Total Carrying Amount      
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]      
Cash and cash equivalents 687.7 645.9  
Derivative financial instruments: Assets 7.6 4.8  
Derivative financial instruments: Liabilities 2.7 4.0  
Debt 1,863.7 1,629.1  
Fair Value Measurements at Reporting Date Using Total Estimated Fair Value      
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]      
Cash and cash equivalents 687.7 645.9  
Derivative financial instruments: Assets 7.6 4.8  
Derivative financial instruments: Liabilities 2.7 4.0  
Debt 2,003.2 $ 1,847.7  
Fair Value Measurements at Reporting Date Using Total Estimated Fair Value | Fair Value Measurements at Reporting Date Using Quoted Prices in Active Markets for Identical Assets (Level 1) | Allegheny Technologies, Convertible Senior Notes, 3.5%, Due 2025 [Member] | Convertible Debt [Member]      
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]      
Debt $ 379.0