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Fair Value of Financial Instruments (Details) - USD ($)
$ in Millions
Sep. 30, 2020
Jul. 02, 2020
Dec. 31, 2019
Carrying Value Reported Amount Fair Value Disclosure      
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]      
Cash and cash equivalents $ 572.2   $ 490.8
Derivative financial instruments, Assets 4.1   5.6
Derivative Financial Instruments, liabilities 4.3   7.5
Debt 1,605.4   1,411.2
Estimate Of Fair Value Fair Value Disclosure      
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]      
Cash and cash equivalents 572.2   490.8
Derivative financial instruments, Assets 4.1   5.6
Derivative Financial Instruments, liabilities 4.3   7.5
Debt 1,597.7   1,676.5
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 572.2   490.8
Derivative financial instruments, Assets 0.0   0.0
Derivative Financial Instruments, liabilities 0.0   0.0
Debt 1,367.9   1,552.8
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 4.1   5.6
Derivative Financial Instruments, liabilities 4.3   7.5
Debt 229.8   123.7
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 $ 49.0 $ 51.4 $ 0.0