Stockholders' equity - Capital stock (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Jun. 20, 2017
Capital stock
Amount of stock repurchased	$ 34
Treasury stock, shares	5,393,331	5,390,107
Treasury stock, value	$ 57,337	$ 57,303
Common Class B
Capital stock
Common stock shares issued, including forfeiture	20,800,998
Common stock shares outstanding, including forfeiture	20,800,998
Common Class A
Capital stock
Common stock shares issued, including forfeiture	20,283,906
Common stock shares outstanding, including forfeiture	20,283,906
Authorized repurchase amount			$ 25,000
Stock repurchase price (in dollars per share)			$ 0.0001
Treasury stock, shares	1,800,000
Treasury stock, value	$ 21,900
Remaining authorization for future repurchases	$ 3,100
Common Class A | Treasury Stock
Capital stock
Number of shares repurchased	3,225
Amount of stock repurchased	$ 34