Convertible Notes - Schedule of Interest Expense Schedule of Interest Expense Related to Convertible Notes (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Debt Instrument [Line Items]
Amortization of discount			$ 7,944	$ 7,616
Convertible Senior Notes Due 2023 [Member]
Debt Instrument [Line Items]
Contractual coupon interest	$ 5,938	$ 5,938	17,813	17,813
Amortization of discount	2,059	1,969	6,087	5,790
Amortization of transaction costs	641	614	1,857	1,826
Total	$ 8,638	$ 8,521	$ 25,757	$ 25,429