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Asset Retirement Obligations (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Jun. 30, 2015
Jun. 30, 2014
Asset Retirement Obligations        
Asset retirement obligations $ 17,737 $ 20,382 $ 21,599 $ 26,308
Changes in Company's asset retirement obligations        
Asset retirement obligations - beginning of period     21,599 26,308
Liabilities incurred     2 844
Liabilities settled       (47)
Liabilities eliminated through asset sales     (4,699) (7,652)
Current period accretion expense 390 432 835 929
Asset retirement obligations - end of period $ 17,737 $ 20,382 $ 17,737 $ 20,382