Income taxes (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Income Taxes
Income tax expense	$ 162,000	$ 103,000	$ 273,000	$ 103,000
Foreign income tax expense	62,000	$ 100,000	200,000	$ 100,000
State income tax expense	100,000		$ 100,000
Effective income tax rate before adjustments (as a percent)			37.00%
Unrecognized tax benefits	5,700,000		$ 5,700,000		$ 4,300,000
Interest and penalties accrued	$ 0		$ 0