Income taxes (Details) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Income Taxes
Income tax expense	$ 44,000	$ 118,000	$ 2,013,000	$ 335,000
Foreign income tax expense	6,000	67,000	2,000,000	192,000
State income tax expense	38,000	$ 51,000	18,000	$ 143,000
Corporate tax rate					21.00%	34.00%
Unrecognized tax benefits	8,300,000		8,300,000		$ 7,400,000
Interest and penalties accrued	$ 0		$ 0