Income Taxes - Tax Effects of Temporary Differences that Give Rise to Deferred Tax Assets (Details 3) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Deferred tax assets:
Net operating loss carryforwards	$ 54,197	$ 46,853
Research credit carryforward	4,527	3,683
Stock options and other	1,474	795
Total gross deferred tax assets	60,198	51,331
Valuation allowance for deferred tax assets	(60,198)	(51,331)
Income taxes
Deferred income tax expense or benefit	0	0
Valuation allowance
Net change in the valuation allowance, related primarily to net operating losses	$ 8,900	$ 4,700