Stock incentive plans - Compensation expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Recognized stock-based compensation expense
Total stock-based compensation expense	$ 2,017	$ 2,054	$ 5,534	$ 6,434
Out-of-period adjustments			481
Stock-based compensation expense capitalized	181	224	497	689
Network operations
Recognized stock-based compensation expense
Total stock-based compensation expense	379	387	1,119	1,241
Development and technology
Recognized stock-based compensation expense
Total stock-based compensation expense	230	325	608	962
Selling and marketing
Recognized stock-based compensation expense
Total stock-based compensation expense	453	578	1,347	1,655
General and administrative
Recognized stock-based compensation expense
Total stock-based compensation expense	$ 955	$ 764	$ 2,460	$ 2,576