Convertible Notes, Net - Summary of Details about Interest Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Discount and issuance cost amortization	$ 21,988	$ 19,339	$ 20,708
Convertible Senior Note [Member]
Debt Instrument [Line Items]
Cash coupon	19,259	9,056	9,056
Discount and issuance cost amortization	4,040	2,736	2,571
Total interest expense	$ 23,299	$ 11,792	$ 11,627