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Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2018
Asset Retirement Obligation Disclosure [Abstract]  
Asset Retirement Obligations Rollforward
The following summarizes changes in the Company’s AROs:
millions
2018

 
2017

Carrying amount at January 1
$
2,794

 
$
2,931

Liabilities acquired
—

 
4

Liabilities incurred
153

 
191

Property dispositions
(99
)
 
(154
)
Liabilities settled
(274
)
 
(135
)
Accretion expense
130

 
144

Revisions in estimated liabilities
395

 
(187
)
Carrying amount at December 31
$
3,099

 
$
2,794