Income Taxes - Details of Significant Components of Net Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Net operating loss carryforwards	$ 13,394	$ 15,337
State income taxes	1	1
Deferred revenue	5,349	458
Research and development and other credits	11,447	11,418
Reserves and accruals recognized in different periods	3,088	5,397
Basis difference in investment	583	969
Capitalized R&D expenses	3,623	4,569
Depreciation and amortization	413	585
Deferred rent	183	306
Other	0	2
Total deferred tax assets	38,081	39,042
Valuation allowance	(37,680)	(38,683)
Net deferred tax assets	401	359
Deferred Tax Liabilities, Foreign Credits	(43)	(33)
Deferred Tax Liabilities, Net	(43)	(33)
Deferred Taxes, Net	$ 358	$ 326