Income taxes (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Income Taxes
Income tax expense	$ 272,000	$ 25,000	$ 648,000	$ 63,000
State income tax expense		$ 0		$ 0
Unrecognized tax benefits	13,600,000		13,600,000		$ 11,500,000
Interest and penalties accrued	0		0
Deferred tax assets acquired	5,800,000		5,800,000
NOL carryforwards subject to expiration	$ 12,100,000		$ 12,100,000