Shareholders' equity - Additional paid-in capital (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Additional paid-in capital
Balance at the beginning of period	$ 114,917	$ 83,548
Share-based compensation expense	981	211
Exercise of stock options	77	25
Balance at the end of period	99,632	73,332
Additional paid-in capital
Additional paid-in capital
Balance at the beginning of period	3,805	2,655
Share-based compensation expense	981	211
Exercise of stock options	(56)	(26)
Balance at the end of period	$ 4,730	$ 2,840