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Fair Value Measurements and Financial Instruments - Marketable Securities and Financial Instruments on Fair Value Basis (Detail) - USD ($)
$ in Millions
Sep. 30, 2017
Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Marketable securities, current $ 0.8 $ 4.4
Marketable securities, noncurrent 256.7 253.5
Interest rate swap   (3.1)
Quoted Prices in Active Markets (Level 1)    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Marketable securities, noncurrent 2.1 0.6
Significant Other Observable Inputs (Level 2)    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Marketable securities, current 0.8 4.4
Marketable securities, noncurrent 4.6 2.9
Interest rate swap   (3.1)
Significant Unobservable Inputs (Level 3)    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Marketable securities, noncurrent $ 250.0 $ 250.0