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Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2015
Asset Retirement Obligation Disclosure [Abstract]  
Change in asset retirement obligations
The following table shows the change in our asset retirement obligations for 2015 and 2014 (dollars in thousands):

 
2015
 
2014
Asset retirement obligations at the beginning of year
$
390,750

 
$
346,729

Changes attributable to:
 

 
 

Accretion expense
25,163

 
23,567

Settlements
(32,048
)
 
(29,497
)
Estimated cash flow revisions
17,556

 
43,899

Newly incurred obligation
42,155

 
6,052

Asset retirement obligations at the end of year
$
443,576

 
$
390,750