Income taxes (Details) - USD ($)	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Income Taxes
Income tax expense	$ 134,000	$ 23,000
State income tax expense	0
Unrecognized tax benefits	12,200,000		$ 11,500,000
Interest and penalties accrued	$ 0