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Asset Retirement Obligations (Tables)
 6 Months Ended
Jun. 30, 2016
Asset Retirement Obligation Disclosure [Abstract]  
Changes in Asset Retirement Obligations
Changes in ARO are as follows:
 
Six Months Ended
June 30,
(millions)
2016
 
2015
Asset Retirement Obligations, Beginning Balance
$
989

 
$
751

Liabilities Incurred
3

 
16

Liabilities Settled
(38
)
 
(15
)
Revision of Estimate
4

 
79

Accretion Expense (1)
25

 
21

Asset Retirement Obligations, Ending Balance
$
983

 
$
852

(1) 
Accretion expense is included in Depreciation, Depletion and Amortization (DD&A) expense in the consolidated statements of operations.