Income Taxes - Reconciliation of Effective Income Tax Rate (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
U.S. federal statutory income tax rate	21.00%	21.00%	21.00%
State income taxes, net of federal benefit	(5.90%)	(5.80%)	(6.20%)
Foreign rate differential	0.00%	(0.10%)	0.00%
Research and development tax credits	(1.10%)	(1.10%)	(3.70%)
Change in fair value of preferred stock warrant liability	0.00%	0.10%	2.40%
Other permanent differences	0.90%	1.40%	0.90%
Change in deferred tax asset valuation allowance	27.20%	26.80%	27.80%
Other	0.10%	(0.30%)	(0.20%)
Effective income tax rate	0.20%	0.00%	0.00%