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Asset Retirement Obligation (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2016
Jun. 30, 2015
Jun. 30, 2016
Jun. 30, 2015
Dec. 31, 2015
Asset Retirement Obligation [Abstract]          
Asset retirement obligation, beginning balance     $ 20,364 $ 17,031 $ 17,031
Liabilities added from acquisitions         3,634
Sold         (58)
Revisions to cost estimates     (1,393)   (1,156)
Settlements     (86)   (186)
Accretion expense $ 315 $ 264 630 $ 517 1,099
Asset retirement obligation, ending balance $ 19,515   $ 19,515   $ 20,364