Income taxes (Details) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Income Taxes
Income tax expense	$ 118,000	$ 162,000	$ 335,000	$ 273,000
Foreign income tax expense	67,000	62,000	192,000	200,000
State income tax expense	51,000	$ 100,000	$ 143,000	$ 100,000
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	7,200,000		$ 7,200,000		$ 5,900,000
Interest and penalties accrued	$ 0		$ 0