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

 
$
253

Liabilities Incurred
23

 
1

Liabilities Settled
(2
)
 
(12
)
Revision of Estimate
20

 
6

Accretion Expense
14

 
10

Other
(89
)
 
—

Asset Retirement Obligations, Ending Balance
$
343

 
$
258