Income taxes (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Income Taxes
Income tax expense	$ 242,000	$ 15,000	$ 376,000	$ 38,000
State income tax expense		$ 0		$ 0
Unrecognized tax benefits	12,900,000		12,900,000		$ 11,500,000
Interest and penalties accrued	$ 0		$ 0