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Asset Retirement Obligation (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Dec. 31, 2016
Asset Retirement Obligation      
Asset retirement obligation, beginning balance $ 13,579 $ 20,364 $ 20,364
Liabilities added from acquisitions 195   912
Sold     (6,291)
Revisions to cost estimates     (2,399)
Settlements     (134)
Accretion expense 258 $ 315 1,127
Asset retirement obligation, ending balance 14,032   13,579
Legally restricted assets $ 0   $ 0