Income Taxes - Reconciliation of U.S. Federal Statutory Income Tax Rate to Effective Income Tax Rate (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
U.S. Statutory Rate	21.00%	34.00%	34.00%
Change in valuation allowance	(20.60%)	56.40%	(11.80%)
State taxes (net of federal benefit)	0.50%	1.90%	(4.40%)
Federal research and development credit	4.10%	7.10%	67.40%
Incentive stock options	(2.90%)	12.50%	(46.20%)
Unrecognized tax benefits	(1.00%)	(3.50%)	(47.10%)
Preferred stock warrant revaluation			12.60%
Impact of Tax Cuts and Jobs Act of 2017		(113.50%)
Return to provision - deductible transaction costs		5.60%
Other, net	(0.40%)	0.10%	(15.50%)
Effective income tax rate	0.70%	0.60%	(11.00%)