Income Taxes - Effective Income Tax Rate Reconciliation (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
U.S. federal income tax at statutory rate	21.00%	21.00%	34.00%
State taxes (net of federal benefit)	4.71%	5.67%	2.40%
Stock-based compensation	1.20%	7.51%	(14.74%)
Permanent differences	(0.97%)	(0.57%)	(0.29%)
Federal research and development credit	(2.45%)	(4.26%)	(7.08%)
Change in valuation allowance	(29.73%)	(37.33%)	(27.79%)
Other nondeductible expenses and others	1.34%	(0.54%)	(0.66%)
Change in valuation allowance for Tax Act impact	0	0	0.8437
Change in deferred balance before valuation allowance for Tax Reform impact	0.00%	0.00%	(84.37%)
Effective income tax rate	0.00%	0.00%	0.00%