Income taxes (Details) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Income Taxes
Income tax expense	$ 1,895,000	$ 113,000	$ 1,969,000	$ 217,000
Foreign income tax expense	1,900,000	65,000	2,000,000	100,000
State income tax expense	20,000	$ 48,000	20,000	$ 92,000
Corporate tax rate					21.00%	34.00%
Unrecognized tax benefits	8,100,000		8,100,000		$ 7,400,000
Interest and penalties accrued	$ 0		$ 0