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Asset Retirement Obligations (Tables)
 3 Months Ended
Mar. 31, 2013
Asset Retirement Obligation Disclosure [Abstract]  
Changes in Asset Retirement Obligations
Changes in ARO are as follows:  
 
Three Months Ended
March 31,
 
2013
 
2012
(millions)
 
 
 
Asset Retirement Obligations, Beginning Balance
$
402

 
$
377

Liabilities Incurred
1

 
6

Liabilities Settled
(10
)
 
(2
)
Revision of Estimate
(1
)
 
3

Accretion Expense (1)
7

 
7

Asset Retirement Obligations, Ending Balance
$
399

 
$
391


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