Income Taxes - Significant Components of the Company's Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred Tax Assets, Net [Abstract]
Fixed assets and intangibles	$ 30,804	$ 0
Net operating loss carryforwards	22,355	68,639
Research credits	5,209	2,643
Stock awards	3,159	2,583
Other	754	1,558
Gross deferred tax assets	62,281	75,423
Valuation allowance	(61,911)	(74,520)
Total deferred tax assets net of valuation allowance	370	903
Deferred tax liabilities	(370)	(903)
Net deferred tax assets	$ 0	$ 0