Income Taxes - Summarized Report of Domestic Federal Statutory Tax Rates to Loss before Income Taxes from Operations (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Abstract]
Federal tax benefit at statutory rate	$ (7,246)	$ (12,489)	$ (13,469)
State tax benefit net of federal effect	(1,243)	(2,143)	(2,311)
Research and development credits	(456)	(560)	(532)
Other permanent differences	8	2	14
Share-based compensation	678	414	650
Adjustment to basis in subsidiary	(10,808)	(3,246)	15,899
Change in valuation allowance	17,140	16,946	(357)
Reduction of deferred tax asset attributed to expired net operating loss carryforward	1,927	1,076	106
Other	0	0	0
Total	$ 0	$ 0	$ 0