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Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2019
Asset Retirement Obligation Disclosure [Abstract]  
Change in asset retirement obligations
The following table shows the change in our asset retirement obligations for 2019 and 2018 (dollars in thousands):

 
2019
 
2018
Asset retirement obligations at the beginning of year
$
726,545

 
$
679,529

Changes attributable to:
 

 
 

Accretion expense
39,726

 
36,876

Settlements
(12,591
)
 
(9,726
)
Estimated cash flow revisions
(96,462
)
 
2,002

Newly incurred or acquired obligations
—

 
17,864

Asset retirement obligations at the end of year
$
657,218

 
$
726,545