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Asset Retirement Obligations (Summary of Changes in Asset Retirement Obligations) (Details) - USD ($)
$ in Thousands
1 Months Ended 12 Months Ended
Dec. 31, 2019
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Asset Retirement Obligations [Abstract]        
Beginning balance   $ 14,816 $ 20,163 $ 18,612
Accretion   812 1,180 951
Additions   595    
Acquisitions and dispositions       (103)
Revisions $ 400 379 6,527 (703)
Ending balance $ 15,844 $ 15,844 $ 14,816 $ 20,163