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Asset Retirement Obligations (Tables)
 9 Months Ended
Sep. 30, 2013
Asset Retirement Obligation Disclosure [Abstract]  
Changes in Asset Retirement Obligations
Changes in ARO are as follows:
 
Nine Months Ended
September 30,
 
2013
 
2012
(millions)
 
 
 
Asset Retirement Obligations, Beginning Balance
$
402

 
$
377

Liabilities Incurred
4

 
24

Liabilities Settled
(15
)
 
(98
)
Revision of Estimate
7

 
30

Accretion Expense (1)
21

 
21

Other
(2
)
 
(34
)
Asset Retirement Obligations, Ending Balance
$
417

 
$
320


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