Income Taxes - Effective Income Tax Rate Reconciliation (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Effective income tax rate reconciliation
U.S. Federal statutory tax rate	(21.00%)	(21.00%)	(21.00%)
Deferred state taxes	(2.30%)	1.40%	(1.90%)
Common stock warrant liability	13.40%		(1.00%)
Other, net	(3.40%)	(0.50%)	0.90%
Change in valuation allowance	8.40%	20.10%	13.20%
Total effective income tax rate	(4.90%)	0.00%	(9.80%)