Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Effective Income Tax Rate Reconciliation, Percent.
Tax at statutory rates	35.00%	35.00%	35.00%
State income taxes	5.20%	6.60%	5.10%
Permanent differences	(1.20%)	(1.30%)	(0.80%)
Research and development credits	1.30%	1.50%	1.90%
Change in valuation allowance	(40.30%)	(41.80%)	(41.20%)
Effective tax rate	0.00%	0.00%	0.00%
Deferred tax assets:
Net operating loss carryforwards	$ 69,411	$ 49,417
Tax credits	5,269	4,107
Other temporary differences	1,636	1,623
Startup expenditures	5,088	5,288
Stock option expenses	2,779	1,649
Total deferred tax assets	84,183	62,084
Deferred tax asset valuation allowance	(83,924)	(61,813)
Net deferred tax assets	259	271
Deferred tax liabilities:
Prepaid expenses	$ (259)	$ (271)