Asset Retirement Obligations - Liability at Period End (Details) - USD ($) $ in Thousands |
Dec. 31, 2016 |
Dec. 31, 2015 |
Dec. 31, 2014 |
|---|---|---|---|
| Asset Retirement Obligations | |||
| ARO liability at end of period | $ 20,058 | $ 20,980 | $ 13,610 |
| Less: Current portion of ARO | (600) | (679) | |
| Total long-term ARO at end of period | $ 19,458 | $ 20,301 |