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Asset Retirement Obligations (Tables)
 9 Months Ended
Sep. 30, 2012
Asset Retirement Obligation Disclosure [Abstract]  
Changes in Asset Retirement Obligations
Changes in asset retirement obligations were as follows: 
 
Nine Months Ended
September 30,
 
2012
 
2011
(millions)
 
 
 
Asset Retirement Obligations, Beginning Balance
$
377

 
$
253

Liabilities Incurred
24

 
2

Liabilities Settled
(98
)
 
(19
)
Revision of Estimate
30

 
9

Accretion Expense
21

 
15

Other
(34
)
 
—

Asset Retirement Obligations, Ending Balance
$
320

 
$
260