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Asset Retirement Obligation (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Dec. 31, 2015
Asset Retirement Obligation          
Asset retirement obligation, beginning balance     $ 20,364 $ 17,031 $ 17,031
Liabilities added from acquisitions         3,634
Sold     (6,512)   (58)
Revisions to cost estimates     (1,313)   (1,156)
Settlements     (142)   (186)
Accretion expense $ 271 $ 265 901 $ 782 1,099
Asset retirement obligation, ending balance $ 13,298   $ 13,298   $ 20,364