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Fair Value of Financial Instruments (Details) - USD ($)
$ in Millions
Mar. 31, 2023
Dec. 31, 2022
Carrying Value Reported Amount Fair Value Disclosure    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Cash and cash equivalents $ 196.2 $ 584.0
Derivative financial instruments, Assets 3.7 18.0
Derivative Financial Instruments, liabilities 6.5 4.6
Debt 1,743.3 1,765.2
Estimate Of Fair Value Fair Value Disclosure    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Cash and cash equivalents 196.2 584.0
Derivative financial instruments, Assets 3.7 18.0
Derivative Financial Instruments, liabilities 6.5 4.6
Debt 2,138.4 1,964.5
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 196.2 584.0
Derivative financial instruments, Assets 0.0 0.0
Derivative Financial Instruments, liabilities 0.0 0.0
Debt 1,861.5 1,665.7
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 3.7 18.0
Derivative Financial Instruments, liabilities 6.5 4.6
Debt 276.9 298.8
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 $ 753.0 $ 590.0