Operating Assets and Liabilities - Other receivables (Details) - USD ($) $ in Thousands |
Dec. 31, 2017 |
Dec. 31, 2016 |
|---|---|---|
| Operating Assets and Liabilities | ||
| VAT receivables | $ 513 | $ 305 |
| Accrued interest income | 117 | |
| Other receivables | 5 | 5 |
| Total | $ 518 | $ 427 |