Income taxes (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Income Taxes
Income tax expense	$ 104,000	$ 47,000
Foreign income tax expense	60,000	47,000
State income tax expense	$ 44,000	$ 0
Effective income tax rate before adjustments (as a percent)	24.00%
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	$ 6,300,000		$ 5,900,000
Interest and penalties accrued	$ 0