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Asset Retirement Obligations (Tables)
 9 Months Ended
Sep. 30, 2017
Asset Retirement Obligation Disclosure [Abstract]  
Changes in Asset Retirement Obligations
Changes in ARO are as follows:
 
Nine Months Ended September 30,
(millions)
2017
 
2016
Asset Retirement Obligations, Beginning Balance
$
935

 
$
989

Liabilities Incurred
83

 
5

Liabilities Settled
(53
)
 
(87
)
Revision of Estimate
(56
)
 
4

Accretion Expense (1)
35

 
37

Asset Retirement Obligations, Ending Balance
$
944

 
$
948


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