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Asset Retirement Obligations (Tables)
 6 Months Ended
Jun. 30, 2015
Asset Retirement Obligation Disclosure [Abstract]  
Changes in Asset Retirement Obligations
Changes in ARO are as follows:
 
Six Months Ended
June 30,
(millions)
2015
 
2014
Asset Retirement Obligations, Beginning Balance
$
751

 
$
586

Liabilities Incurred
16

 
22

Liabilities Settled
(15
)
 
(43
)
Revision of Estimate
79

 
120

Accretion Expense (1)
21

 
19

Asset Retirement Obligations, Ending Balance
$
852

 
$
704

(1) Accretion expense is included in DD&A expense in the consolidated statements of operations.