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Fair Value of Financial Instruments (Details) - USD ($)
$ in Millions
Sep. 30, 2022
Dec. 31, 2021
Carrying Value Reported Amount Fair Value Disclosure    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Cash and cash equivalents $ 329.1 $ 687.7
Derivative financial instruments, Assets 18.6 7.6
Derivative Financial Instruments, liabilities 3.8 2.7
Debt 1,747.3 1,863.7
Estimate Of Fair Value Fair Value Disclosure    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Cash and cash equivalents 329.1 687.7
Derivative financial instruments, Assets 18.6 7.6
Derivative Financial Instruments, liabilities 3.8 2.7
Debt 1,839.6 2,003.2
Estimate Of Fair Value Fair Value Disclosure | Fair Value Inputs Level 1    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Cash and cash equivalents 329.1 687.7
Derivative financial instruments, Assets 0.0 0.0
Derivative Financial Instruments, liabilities 0.0 0.0
Debt 1,558.7 1,690.1
Estimate Of Fair Value Fair Value Disclosure | Fair Value 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 18.6 7.6
Derivative Financial Instruments, liabilities 3.8 2.7
Debt 280.9 313.1
Convertible Debt | ATI Inc. 3.5% Convertible Senior Notes due 2025 | Estimate Of Fair Value Fair Value Disclosure | Fair Value Inputs Level 1    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Debt $ 535.0 $ 379.0