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Financial Instruments - Financial Instruments not Carried at Fair Value but which Require Fair Value Disclosure (Detail) - USD ($)
$ in Millions
Sep. 30, 2021
Dec. 31, 2020
Sep. 30, 2020
Dec. 31, 2019
Fair Value Disclosures [Abstract]        
Cash, cash equivalents and restricted cash, Carrying amount $ 379.7 $ 362.0 $ 353.9 $ 392.3
Long-term debt - Fixed rate Senior Notes, Carrying amount 460.4 485.7    
Cash, cash equivalents and restricted cash, Fair value 379.7 362.0    
Long-term debt - Fixed rate Senior Notes, Fair value $ 472.4 $ 499.9