Income Taxes - Components of Net Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred Tax Asset
Net Operating Loss	$ 37,168	$ 55,521
Tax Credits	1,846	2,034
Stock Based Compensation	900	997
Intangibles	489	843
Other	290	337
Subtotal	40,693	59,732
Valuation Allowance	(39,072)	(58,983)
Total Deferred Tax Assets	1,621	749
Fixed Assets	(1,621)	(749)
Total Deferred Tax Liabilities	(1,621)	(749)
Net Deferred Tax Asset	$ 0	$ 0