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Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2018
Asset Retirement Obligation Disclosure [Abstract]  
Change in asset retirement obligations The following table shows the change in our asset retirement obligations for 2018 and 2017 (dollars in thousands):

 
2018
 
2017
Asset retirement obligations at the beginning of year
$
679,529

 
$
624,475

Changes attributable to:
 

 
 

Accretion expense
36,876

 
33,104

Settlements
(9,726
)
 
—

Estimated cash flow revisions
2,002

 
21,950

Newly incurred or acquired obligations
17,864

 
—

Asset retirement obligations at the end of year
$
726,545

 
$
679,529