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Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2017
Asset Retirement Obligation Disclosure [Abstract]  
Schedule of Change in Asset Retirement Obligation
Changes in asset retirement obligations were as follows:
 
For Year Ended December 31,
(In millions)
2017
 
2016
Beginning balance
$
1,652

 
$
1,544

Incurred liabilities, including acquisitions
25

 
14

Settled liabilities, including dispositions
(50
)
 
(74
)
Accretion expense (included in depreciation, depletion and amortization)
85

 
79

Revisions of estimates
(227
)
 
96

Held for sale
(2
)
 
(7
)
Ending balance
$
1,483

 
$
1,652