Stock incentive plans - Compensation expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Recognized stock-based compensation expense
Total stock-based compensation expense	$ 2,946	$ 4,288	$ 6,072	$ 7,332
Stock-based compensation expense capitalized	206	173	392	367
Network operations
Recognized stock-based compensation expense
Total stock-based compensation expense	516	562	1,053	1,108
Development and technology
Recognized stock-based compensation expense
Total stock-based compensation expense	314	280	592	550
Selling and marketing
Recognized stock-based compensation expense
Total stock-based compensation expense	393	639	866	1,065
General and administrative
Recognized stock-based compensation expense
Total stock-based compensation expense	$ 1,723	$ 2,807	$ 3,561	$ 4,609