Summary of Senior Subordinated Convertible Obligations (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Aug. 31, 2017
Debt Instrument [Line Items]
Convertible Notes	$ 870,000
Convertible notes, current	361,882	$ 0
Convertible Notes, net of unamortized discount and deferred offering costs	486,238	830,417
Total convertible debt, net	848,120	830,417
Convertible Notes, fair value	927,518	911,348
1.50% Senior Subordinated Convertible Notes Due in October 2020
Debt Instrument [Line Items]
Convertible Notes	374,993	374,993
Convertible debt, unamortized discount	(12,078)	(26,581)
Convertible debt, Unamortized deferred offering costs	(1,033)	(2,334)
Convertible notes, current	361,882
Convertible Notes, net of unamortized discount and deferred offering costs		346,078
Convertible Notes, fair value	405,679	419,722
0.599% Senior Subordinated Convertible Notes Due in August 2024
Debt Instrument [Line Items]
Convertible Notes	495,000	495,000
Convertible debt, unamortized discount	(6,533)	(7,946)	$ (9,900)
Convertible debt, Unamortized deferred offering costs	(2,229)	(2,715)
Convertible Notes, net of unamortized discount and deferred offering costs	486,238	484,339
Convertible Notes, fair value	$ 521,839	$ 491,626