Debt - Summary of Interest Expense on Convertible Debt (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Schedule Of Interest Expenses [Line Items]
Total interest expense on convertible debt	$ 2,937	$ 12,131	$ 16,530	$ 35,918
Convertible Senior Notes
Schedule Of Interest Expenses [Line Items]
Coupon interest expense	2,271	3,331	6,681	9,819
Amortization of debt issuance costs	508	1,008	1,522	3,018
Accretion of discount on convertible notes	4,005	7,792	11,860	23,081
Total interest expense on convertible debt	$ 6,784	$ 12,131	$ 20,063	$ 35,918