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Fair Value of Financial Instruments (Details) - USD ($)
$ in Millions
Jun. 30, 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 $ 539.1 $ 490.8
Derivative financial instruments, Assets 1.5 5.6
Derivative Financial Instruments, liabilities 8.0 7.5
Debt 1,596.1 1,411.2
Estimate Of Fair Value Fair Value Disclosure    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Cash and cash equivalents 539.1 490.8
Derivative financial instruments, Assets 1.5 5.6
Derivative Financial Instruments, liabilities 8.0 7.5
Debt 1,563.8 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 539.1 490.8
Derivative financial instruments, Assets 0.0 0.0
Derivative Financial Instruments, liabilities 0.0 0.0
Debt 1,336.9 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 1.5 5.6
Derivative Financial Instruments, liabilities 8.0 7.5
Debt 226.9 123.7
Convertible Debt | Allegheny Technologies, Convertible Senior Notes, 3.5%, Due 2025    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Equity component of convertible debt $ 50.3 $ 0.0