Tax Effects of Temporary Differences that Give Rise to Net Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Deferred tax assets:
Net operating loss carryforwards	$ 133,514	$ 123,647	$ 115,870
Research and other credits	3,482	5,610	6,047
Other temporary differences	2,319	4,804	(2,478)
Deferred tax assets - before valuation allowance	139,315	134,061	119,439
Valuation allowance	$ (139,315)	$ (134,061)	$ (119,439)