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0.25% Convertible Senior Notes, Convertible Note Hedge and Warrant - Additional Information (Detail) - 0.25% Convertible Senior Notes Due 2022 [Member] - USD ($)
3 Months Ended
Mar. 31, 2018
May 31, 2017
Debt Instrument [Line Items]    
Debt instrument, interest rate, stated percentage   0.25%
Debt instrument, maturity date Jun. 01, 2022  
Private Offering [Member]    
Debt Instrument [Line Items]    
Principal amount of long term debt   $ 350,000,000
Over-Allotment Options [Member]    
Debt Instrument [Line Items]    
Principal amount of long term debt   $ 50,000,000