Income Taxes - Effective Tax Rate Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Reconciliation of the U.S. statutory income tax rate to the effective income tax rate
Federal income tax (benefit) at statutory rate	21.00%	34.00%
Permanent differences	(0.68%)	(1.16%)
Federal research and development credits and adjustments	4.25%	2.36%
State income tax benefit, net of federal benefit	6.84%	5.80%
Tax reform deferred rate change		(72.21)
Other (as a percent)	(1.41%)	(0.36%)
Change in valuation allowance	(30.00%)	31.57%
Deferred tax assets
Net operating loss carryforwards	$ 48,107	$ 41,436
Tax credit carryforwards	7,174	5,896
Capitalized research and development	1,394	1,879
Capitalized legal expenses	1,132	1,156
Other differences	1,370	1,048
Total deferred tax assets	59,177	51,415
Valuation allowance	(59,177)	(51,415)
Valuation allowance
Recorded increase (decrease) in valuation allowance	7,800	$ (8,400)
Federal
Valuation allowance
Net operating loss	176,300
Net operating loss carryforwards that will expire from 2023 through 2037	150,500
Net operating loss carryforwards, carried forward indefinitely	25,800
Tax credit carryforwards	5,600
State
Valuation allowance
Net operating loss	175,400
Net operating loss carryforwards that will expire from 2023 through 2037	175,400
Tax credit carryforwards	$ 2,000