Stockholders' equity - Capital Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended		24 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Dec. 31, 2018	Aug. 15, 2018	Jun. 20, 2017
Capital Stock
Amount of stock repurchased	$ 135	$ 1,521	$ 648	$ 1,555
Common Class B
Capital Stock
Common stock, shares issued	19,720,381		19,720,381		19,720,381	19,720,381
Common stock, shares outstanding	19,720,381		19,720,381		19,720,381	19,720,381
Common Class A
Capital Stock
Common stock, shares issued excluding treasury shares	19,749,173		19,749,173		19,749,173
Common stock, shares issued	25,033,981		25,033,981		25,033,981	24,849,589
Common stock, shares outstanding	25,033,981		25,033,981		25,033,981	24,849,589
Stock repurchase price (in dollars per share)								$ 0.0001
Common Class A | Treasury Stock
Capital Stock
Number of shares repurchased			51,227		2,000,000
Amount of stock repurchased	$ 135	$ 1,521	$ 648	$ 1,555	$ 25,000
Common Class A | Maximum
Capital Stock
Authorized repurchase amount							$ 25,000	$ 25,000