Income Taxes - Significant Components of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Deferred revenue		$ 7,377
Basis difference in equipment	$ (587)	(1,084)
Basis difference in intangibles	(8,288)	1,201
Accrued liabilities and other	654	550
R&D credit	11,882	11,589
AMT credit	79	79
Stock options	6,115	5,697
Net operating loss carry-forward	61,660	83,621
Gross, Deferred Tax Assets	71,515	109,030
Less: valuation allowance	(71,515)	(109,030)
Net of Valuation Allowance	$ 0	$ 0