Income taxes - Reconciliation of Beginning and Ending Amount of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands |
12 Months Ended | ||
|---|---|---|---|
Dec. 31, 2016 |
Dec. 31, 2015 |
Dec. 31, 2014 |
|
| Reconciliation of unrecognized tax benefits | |||
| Beginning balance | $ 14,943 | $ 8,356 | $ 9,033 |
| Additions based on tax positions related to prior years | 3,140 | 6,448 | 0 |
| Additions based on tax positions related to current year | 256 | 261 | 0 |
| Reductions based on lapse of statute of limitations | (102) | (122) | (215) |
| Reductions based on tax positions related to prior years | 0 | 0 | (462) |
| Ending balance | $ 18,237 | $ 14,943 | $ 8,356 |