Convertible Notes, Net - Summary of Details about Interest Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Discount and issuance cost amortization	$ 28,097	$ 21,988	$ 19,339
Convertible Senior Note [Member]
Debt Instrument [Line Items]
Cash coupon	33,859	19,259	9,056
Discount and issuance cost amortization	5,531	4,040	2,736
Total interest expense	$ 39,390	$ 23,299	$ 11,792