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Asset Retirement Obligation (Tables)
 12 Months Ended
Dec. 31, 2015
Asset Retirement Obligation Disclosure [Abstract]  
Asset Retirement Obligation
 
 
2015
 
2014
 
 
(In millions)
Asset retirement obligation at beginning of year
 
$
2,952

 
$
3,222

Liabilities incurred
 
68

 
171

Liabilities divested
 
(490
)
 
(471
)
Liabilities settled
 
(90
)
 
(146
)
Accretion expense
 
158

 
181

Revisions in estimated liabilities
 
—

 
128

Liabilities held for sale (1)
 
—

 
(133
)
Asset retirement obligation at end of year
 
2,598

 
2,952

Less current portion
 
(36
)
 
(37
)
Asset retirement obligation, long-term
 
$
2,562

 
$
2,915