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Asset Retirement Obligations (Tables)
 9 Months Ended
Sep. 30, 2018
Asset Retirement Obligation Disclosure [Abstract]  
Changes in Asset Retirement Obligations Changes in ARO are as follows:
 
Nine Months Ended September 30,
(millions)
2018
 
2017
Asset Retirement Obligations, Beginning Balance
$
875

 
$
935

Liabilities Incurred
16

 
83

Liabilities Settled
(309
)
 
(53
)
Revisions of Estimates
67

 
(56
)
Accretion Expense (1)
25

 
35

Asset Retirement Obligations, Ending Balance
$
674

 
$
944


(1) 
Accretion expense is included in depreciation, depletion and amortization (DD&A) expense in the consolidated statements of operations.