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Asset Retirement Obligation (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2016
Mar. 31, 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 (26)   (1,156)
Settlements (17)   (186)
Accretion expense 315 $ 253 1,099
Asset retirement obligation, ending balance $ 20,636   $ 20,364