Income Taxes - Summary of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Net operating loss carryforwards	$ 21,627	$ 31,618
Credit carryforwards	4,230	3,200
Stock-based compensation	2,806	1,801
Accruals and reserves	3,953	4,588
Gross deferred tax assets	32,616	41,207
Valuation allowance	(29,818)	(38,307)
Total deferred tax assets, net of valuation allowance	2,798	2,900
Deferred tax liabilities:
Intangible assets	(847)	(1,419)
Prepaid expenses	(1,951)	(1,481)
Total deferred tax liabilities	(2,798)	(2,900)
Net deferred tax assets and liabilities	$ 0	$ 0