Stock incentive plans - Compensation expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Recognized stock-based compensation expense
Total stock-based compensation expense	$ 12,268	$ 14,215	$ 12,805
Stock-based compensation expense capitalized	789	696	727
Network operations
Recognized stock-based compensation expense
Total stock-based compensation expense	2,070	2,174	2,144
Development and technology
Recognized stock-based compensation expense
Total stock-based compensation expense	1,242	1,068	1,070
Selling and marketing
Recognized stock-based compensation expense
Total stock-based compensation expense	1,868	2,060	1,842
General and administrative
Recognized stock-based compensation expense
Total stock-based compensation expense	$ 7,088	$ 8,913	$ 7,749