Stock incentive plans - Compensation expense (Details) - USD ($) $ in Thousands |
12 Months Ended | ||
|---|---|---|---|
Dec. 31, 2017 |
Dec. 31, 2016 |
Dec. 31, 2015 |
|
| Recognized stock-based compensation expense | |||
| Total stock-based compensation | $ 14,215 | $ 12,805 | $ 9,398 |
| Stock-based compensation expense capitalized | 696 | 727 | 778 |
| Network operations | |||
| Recognized stock-based compensation expense | |||
| Total stock-based compensation | 2,174 | 2,144 | 1,504 |
| Development and technology | |||
| Recognized stock-based compensation expense | |||
| Total stock-based compensation | 1,068 | 1,070 | 731 |
| Selling and marketing | |||
| Recognized stock-based compensation expense | |||
| Total stock-based compensation | 2,060 | 1,842 | 3,411 |
| General and administrative | |||
| Recognized stock-based compensation expense | |||
| Total stock-based compensation | $ 8,913 | $ 7,749 | $ 3,752 |