Stock incentive plans - Compensation expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Recognized stock-based compensation expense
Total stock-based compensation expense	$ 1,980	$ 2,036	$ 3,517	$ 4,380
Out-of-period adjustments			481
Stock-based compensation expense capitalized	167	235	316	465
Network operations
Recognized stock-based compensation expense
Total stock-based compensation expense	392	348	740	854
Development and technology
Recognized stock-based compensation expense
Total stock-based compensation expense	220	334	378	637
Selling and marketing
Recognized stock-based compensation expense
Total stock-based compensation expense	472	548	894	1,077
General and administrative
Recognized stock-based compensation expense
Total stock-based compensation expense	$ 896	$ 806	$ 1,505	$ 1,812