Income Taxes - Reconciliation of U.S. Federal Statutory Income Tax Rate to Effective Income Tax Rate (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
U.S. Statutory Rate	34.00%	34.00%	34.00%
Change in valuation allowance	56.40%	(11.80%)	9.90%
State taxes (net of federal benefit)	1.90%	(4.40%)	3.80%
Federal research and development credit	7.10%	67.40%	(60.80%)
Incentive stock options	12.50%	(46.20%)	19.50%
Unrecognized tax benefits	(3.50%)	(47.10%)	28.80%
Preferred stock warrant revaluation		12.60%	(22.40%)
Impact of Tax Cuts and Jobs Act of 2017	(113.50%)
Return to provision - deductible transaction costs	5.60%
Other, net	0.10%	(15.50%)	2.90%
Effective income tax rate	0.60%	(11.00%)	15.70%