New Revenue Recognition Standard - Condensed Consolidated Cash Flows (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net loss	$ (49,221)	$ (68,718)	$ 13,744	$ (15,816)	$ (28,556)	$ (18,293)	$ (20,852)	$ (23,547)	$ (120,011)	$ (91,248)	$ (69,770)
Changes in operating assets and liabilities:
Accounts receivable, net									2,844	7,694	(9,494)
Deferred revenue									0	$ 593	$ (2,985)
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net loss									1,085
Changes in operating assets and liabilities:
Accounts receivable, net									127
Deferred revenue									(958)
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net loss									(118,926)
Changes in operating assets and liabilities:
Accounts receivable, net									(2,717)
Deferred revenue									$ (958)