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Asset Retirement Obligations (ARO) (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 Roll Forward          
Balance at beginning of period     $ 24,345 $ 22,245 $ 22,245
Accretion expense $ 412 $ 379 825 $ 757 1,521
Liabilities incurred     6   579
Settlement of liabilities     (164)   0
Balance at end of period $ 25,012   $ 25,012   $ 24,345