Income taxes (Details) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Income Taxes
Income tax expense	$ 113,000	$ 64,000	$ 217,000	$ 111,000
Foreign income tax expense	65,000	64,000	125,000	100,000
State income tax expense	48,000	$ 0	$ 92,000	$ 0
Corporate tax rate			21.00%		34.00%
Decrease related to deferred tax assets					$ 42,100,000
Increase related to deferred tax liabilities					200,000
Valuation allowance reduction					42,200,000
Unrecognized tax benefits	6,700,000		$ 6,700,000		$ 5,900,000
Interest and penalties accrued	$ 0		$ 0