Income Taxes - Effective Tax Rate Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reconciliation of the U.S. statutory income tax rate to the effective income tax rate
Federal income tax (benefit) at statutory rate	34.00%	34.00%	34.00%
Permanent differences	(0.85%)	(0.86%)	(1.29%)
Federal research and development credits and adjustments	2.43%	2.64%	2.70%
State income tax benefit, net of federal benefit	5.70%	5.77%	6.03%
Other (as a percent)	(0.40%)	0.28%	(0.15%)
Change in valuation allowance	(40.88%)	(41.82%)	(41.29%)
Deferred tax assets
Net operating loss carryforwards	$ 48,682	$ 35,042
Tax credit carryforwards	4,833	3,762
Capitalized research and development	3,394	4,087
Capitalized legal expenses	1,597	1,464
Other differences	1,335	815
Total deferred tax assets	59,841	45,170
Valuation allowance	(59,841)	(45,170)
Valuation allowance
Recorded increase in valuation allowance	14,700	$ 13,600	$ 9,000
Excess tax benefit from exercise of stock options included in federal and state net operating loss	500
Federal
Valuation allowance
Net operating loss carryforwards	124,700
Tax credit carryforwards	3,900
State
Valuation allowance
Net operating loss carryforwards	123,100
Tax credit carryforwards	$ 1,500