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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, 2020
Dec. 31, 2019
Sep. 30, 2019
Dec. 31, 2018
Fair Value Disclosures [Abstract]        
Cash, cash equivalents and restricted cash, Carrying amount $ 353.9 $ 392.3 $ 386.9 $ 374.7
Long-term debt - Fixed rate Senior Notes, Carrying amount 464.7 442.6    
Cash, cash equivalents and restricted cash, Fair value 353.9 392.3    
Long-term debt - Fixed rate Senior Notes, Fair value $ 465.0 $ 457.0