Income Taxes - Components of Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Net operating loss carry forwards	$ 102,942	$ 106,560
Equity-based compensation	7,073	6,937
Other temporary differences	1,169	1,235
Research and development credit and carry forwards	9,079	7,682
Deferred tax assets	120,263	122,414
Less valuation allowance	(120,263)	(122,414)
Net deferred tax assets	$ 0	$ 0