Stockholders' equity - Capital stock (Details) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended
	Sep. 30, 2018	Aug. 15, 2018	Dec. 31, 2017	Jun. 20, 2017
Capital stock
Amount of stock repurchased	$ 2,104
Treasury stock, shares	5,591,680		5,390,107
Treasury stock, value	$ 59,747		$ 57,303
Common Class B
Capital stock
Common stock shares issued, including forfeiture	19,720,381
Common stock shares outstanding, including forfeiture	19,720,381
Common Class A
Capital stock
Common stock shares issued, including forfeiture	19,628,969
Common stock shares outstanding, including forfeiture	19,628,969
Authorized repurchase amount				$ 25,000
Stock repurchase price (in dollars per share)				$ 0.0001
Treasury stock, shares	1,900,000
Treasury stock, value	$ 24,000
Remaining authorization for future repurchases	$ 1,000
Additional authorized repurchase amount		$ 25,000
Common Class A | Treasury Stock
Capital stock
Number of shares repurchased	172,105
Amount of stock repurchased	$ 2,104