Income Taxes - Summary of Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Net operating loss carryforwards	$ 111,819	$ 102,963
Deferred revenue	1,593	2,240
Accrued bonus	3,609	2,963
Deferred rent	3,400	4,137
Nonaccrual receivables	18,510	8,437
Other	3,127	3,509
Research and development credits	17,067	12,512
Gross deferred tax assets	159,125	136,761
Deferred tax liability	(40)	(3,124)
Valuation allowance	(159,085)	(133,637)
Net deferred tax assets	$ 0	$ 0