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Asset Retirement Obligations Asset Retirement Obligations (Narrative) (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2019
Asset Retirement Obligation Disclosure [Abstract]    
Reduction in asset retirement obligation $ 5,400  
Liabilities settled during the period 2,400 $ 3,587
Revision of estimated cash flows (3,000) $ 2,986
Amount transferred to escrow 3,400  
Cash paid to settle 2,400  
Loss recognized on transaction $ 400