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Fair Value Measurements - Carrying Value and Fair Value of Debt (Detail) - USD ($)
$ in Millions
Sep. 30, 2020
Dec. 31, 2019
Carrying Value    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Total Debt $ 2,086.5 $ 2,184.3
Carrying Value | Senior Notes [Member]    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Total Debt 1,786.5 2,184.3
Carrying Value | 2019 Revolving Credit Agreement [Member]    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Total Debt 300.0  
Fair Value    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Total Debt 2,292.4 2,271.4
Fair Value | Senior Notes [Member]    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Total Debt 1,992.4 $ 2,271.4
Fair Value | 2019 Revolving Credit Agreement [Member]    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Total Debt $ 300.0