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Asset Retirement Obligations (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Asset Retirement Obligation, Roll Forward Analysis          
Balance at beginning of period     $ 87,306    
Obligations incurred with development activities     1,517    
Business Acquisition Purchase Price Allocation Asset Retirement Obligation $ 4,687   4,687    
Accretion expense 1,285 $ 1,666 2,573 $ 3,434  
Asset Retirement Obligation, Revision of Estimate     42    
Obligations discharged asset retirements     (6,617)    
Balance end of period 89,508   89,508    
Less current portion (15,959)   (15,959)    
Asset retirement obligations $ 73,549   $ 73,549   $ 71,006