Revenue - Contract balances (Details) - USD ($) $ in Thousands |
Dec. 31, 2018 |
Dec. 31, 2017 |
|---|---|---|
| Revenue | ||
| Accounts receivable | $ 37,347 | $ 24,179 |
| Contract assets | 788 | |
| Contract liabilities | $ 40,476 | |
| Calculated under Revenue Guidance in Effect before Topic 606 | ||
| Revenue | ||
| Accounts receivable | 24,179 | |
| Contract assets | 1,141 | |
| Contract liabilities | $ 35,171 |