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0.25% Convertible Senior Notes, Convertible Note Hedge and Warrant - Schedule of Interest Expense (Detail) - 0.25% Convertible Senior Notes Due 2022 [Member] - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Debt Instrument [Line Items]    
Contractual interest expense $ 1,000 $ 639
Amortization of debt discount 18,923 11,507
Amortization of issuance costs 1,412 859
Total interest expense $ 21,335 $ 13,005