Shareholders’ equity (deficit) - Additional paid-in capital (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Additional paid-in capital
Balance at the beginning of period			$ (55,210)				$ (55,210)
Share-based compensation expense	$ 350	$ 276	211	$ 85	$ 79	$ 125
Exercise of stock options			25		7	30
Balance at the end of period	132,832						132,832
Additional paid-in capital
Additional paid-in capital
Balance at the beginning of period	3,116	2,840	2,655	2,544	2,472	2,372	2,655	$ 2,372
Share-based compensation expense	350	276	211	85	79	125	837	289
Exercise of stock options			(26)		(7)	(25)	(26)	(32)
Balance at the end of period	$ 3,466	$ 3,116	$ 2,840	$ 2,629	$ 2,544	$ 2,472	$ 3,466	$ 2,629