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Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2014
Asset Retirement Obligation Disclosure [Abstract]  
Asset Retirement Obligations Rollforward
The following summarizes changes in the Company’s AROs during 2014 and 2013:
millions
2014
 
2013
Carrying amount of asset retirement obligations at January 1
$
2,022

 
$
1,885

Liabilities incurred
119

 
182

Property dispositions
(70
)
 
(76
)
Liabilities settled
(443
)
 
(162
)
Accretion expense
93

 
110

Revisions in estimated liabilities
332

 
83

Carrying amount of asset retirement obligations at December 31
$
2,053

 
$
2,022