Income Taxes - Reconciliation of provision (benefit) for income taxes (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Taxes
Benefit for income taxes at statutory rate	19.00%	19.00%	19.00%
(Decreases) increases resulting from:
Federal tax credits	1.30%	1.10%	0.40%
Change in valuation allowance	(8.00%)	(7.20%)	(9.40%)
Net losses surrendered for research credit	(5.30%)	(3.70%)	(6.70%)
Preferred share warrants	(3.30%)	(0.60%)	(1.10%)
Other	(2.90%)	(6.80%)	(2.10%)
Effective income tax rate	0.80%	1.80%	0.10%