Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reconciliation from 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	(0.86%)	(1.29%)	(0.50%)
Federal research and development credits and adjustments	2.64%	2.70%	5.80%
State income tax benefit, net of federal benefit	5.77%	6.03%	5.88%
Other (as a percent)	0.28%	(0.15%)	(0.01%)
Change in valuation allowance	(41.82%)	(41.29%)	(45.17%)
Deferred Tax Assets
Net operating loss carryforwards	$ 35,042	$ 22,577
Tax credit carryforwards	3,762	2,748
Capitalized research and development	4,087	4,780
Capitalized legal expenses	1,464	1,223
Other differences	815	195
Total deferred tax assets	45,170	31,523
Valuation allowance	(45,170)	(31,523)
Valuation allowance
Recorded increase in valuation allowance	13,600	$ 9,000	$ 8,200
Excess tax benefit from exercise of stock options included in federal and state net operating loss	400
Federal
Valuation allowance
Net operating loss carryforwards	89,800
Tax credit carryforwards	3,000
Federal | Additional Paid-in Capital
Valuation allowance
Net operating loss carryforwards	400
State
Valuation allowance
Net operating loss carryforwards	88,400
Tax credit carryforwards	1,200
State | Additional Paid-in Capital
Valuation allowance
Net operating loss carryforwards	$ 400