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Asset Retirement Obligations (ARO Activity) (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2019
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]    
Balance at December 31, 2018   $ 37,703
Liabilities settled during the period $ (2,400) (3,587)
Accretion expense   1,258
Revision of estimated cash flows 3,000 (2,986)
Balance at June 30, 2019 $ 32,388 $ 32,388