Income Taxes - Summary of Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Net operating losses	$ 59,756	$ 86,071
Credit carryovers	10,527	9,952
Depreciation and amortization	10,893	15,278
Deferred revenue	634	0
Accruals and reserves	450	701
Capital loss carryover	0	85
Other	1,350	2,677
Total deferred tax assets	83,610	114,764
Less valuation allowance	(83,610)	(114,764)
Net deferred tax assets	$ 0	$ 0