Income Taxes - Reconciliation of income tax (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Reconciliation of income tax:
Income tax (benefit) provision at federal statutory rate (as a percent)	21.00%	21.00%
State income tax, net of federal benefit (as a percent)	6.30%	6.30%
Acquired in-process research and development expense	(15.60%)
Valuation allowance	(10.40%)	(27.90%)
Convertible notes (as a percent)	(1.60%)
Research credits (as a percent)	0.80%	1.00%
Other (as a percent)	(0.50%)	(0.40%)