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Fair Value of Financial Instruments (Details) - USD ($)
$ in Millions
Sep. 30, 2021
Dec. 31, 2020
Jul. 02, 2020
Carrying Value Reported Amount Fair Value Disclosure      
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]      
Cash and cash equivalents $ 1,006.8 $ 645.9  
Derivative financial instruments, Assets 12.2 4.8  
Derivative Financial Instruments, liabilities 3.6 4.0  
Debt 2,313.3 1,629.1  
Estimate Of Fair Value Fair Value Disclosure      
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]      
Cash and cash equivalents 1,006.8 645.9  
Derivative financial instruments, Assets 12.2 4.8  
Derivative Financial Instruments, liabilities 3.6 4.0  
Debt 2,558.7 1,847.7  
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 1,006.8 645.9  
Derivative financial instruments, Assets 0.0 0.0  
Derivative Financial Instruments, liabilities 0.0 0.0  
Debt 2,296.0 1,594.2  
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 12.2 4.8  
Derivative Financial Instruments, liabilities 3.6 4.0  
Debt 262.7 253.5  
Convertible Debt | Allegheny Technologies 3.5% Convertible Senior Notes due 2025      
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]      
Equity component of convertible debt 0.0 $ 46.8 $ 51.4
Convertible Debt | Allegheny Technologies 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 $ 402.0