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Asset Retirement Obligations (Tables)
 6 Months Ended
Jun. 30, 2013
Asset Retirement Obligation Disclosure [Abstract]  
Changes in Asset Retirement Obligations
Changes in ARO are as follows:
 
Six Months Ended
June 30,
 
2013
 
2012
(millions)
 
 
 
Asset Retirement Obligations, Beginning Balance
$
402

 
$
377

Liabilities Incurred
2

 
23

Liabilities Settled
(10
)
 
(2
)
Revision of Estimate
7

 
20

Accretion Expense (1)
14

 
14

Other
—

 
(89
)
Asset Retirement Obligations, Ending Balance
$
415

 
$
343


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