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0.25% Convertible Senior Notes, Convertible Note Hedge and Warrant - Schedule of Net Carrying Amount of Liability Component (Detail) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Debt Instrument [Line Items]    
Net carrying amount $ 303,355 $ 298,447
0.25% Convertible Senior Notes Due 2022 as Liability Component [Member]    
Debt Instrument [Line Items]    
Principal 400,000 400,000
Unamortized debt discount (89,931) (94,498)
Unamortized issuance costs (6,714) (7,055)
Net carrying amount $ 303,355 $ 298,447