Operating Assets and Liabilities - Prepaid expenses (Details) - USD ($) $ in Thousands |
Dec. 31, 2017 |
Dec. 31, 2016 |
|---|---|---|
| Operating Assets and Liabilities | ||
| Advanced payments to contract research and manufacturing organizations | $ 132 | |
| Insurance | $ 421 | 450 |
| Other | 81 | 74 |
| Total | $ 502 | $ 656 |