Income Taxes - Reconciliation Between Provision for Income Taxes and Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Computed “expected” tax benefit	$ (4,425)	$ (3,051)	$ (3,152)
State income taxes, net of federal benefit	(268)	(534)	0
Tax effect of:
Change in valuation allowance	(31,665)	(6,030)	(4,069)
Impact of Tax Cuts and Jobs Act	34,439	0	0
Rate change	0	0	3,524
Expiration of prior year credits and net operating losses	1,256	692	786
Stock compensation	458	491	376
Uncertain tax positions	276	9,010	3,074
Other	(71)	(578)	(539)
Provision for income taxes	$ 0	$ 0	$ 0