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Fair Value Measurements and Financial Instruments - Marketable Securities and Financial Instruments on Fair Value Basis (Detail) - USD ($)
$ in Millions
Jun. 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 257.2 253.5
Interest rate swap (2.8) (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 2.5 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 4.7 2.9
Interest rate swap (2.8) (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