Income taxes (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Income Taxes
Income tax expense	$ 15,000	$ 1,895,000	$ 38,000	$ 1,969,000
State income tax expense	0		0
Unrecognized tax benefits	9,300,000		9,300,000		$ 8,600,000
Interest and penalties accrued	$ 0		$ 0