Income taxes (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Income Taxes
Income tax expense	$ 74,000	$ 104,000
Foreign income tax expense	74,000	60,000
State income tax expense	$ 0	$ 44,000
Corporate tax rate	34.00%		34.00%	21.00%
Unrecognized tax benefits	$ 7,800,000		$ 7,400,000
Interest and penalties accrued	$ 0