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Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2015
Asset Retirement Obligation Disclosure [Abstract]  
Asset Retirement Obligations Rollforward
The following summarizes changes in the Company’s AROs:
millions
2015
 
2014
Carrying amount of asset retirement obligations at January 1
$
2,053

 
$
2,022

Liabilities incurred
104

 
119

Property dispositions
(108
)
 
(70
)
Liabilities settled
(298
)
 
(443
)
Accretion expense
102

 
93

Revisions in estimated liabilities
206

 
332

Carrying amount of asset retirement obligations at December 31
$
2,059

 
$
2,053