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Goodwill (Tables)
 12 Months Ended
Dec. 31, 2012
Goodwill [Abstract]  
Changes in the carrying amount of goodwill
Changes in the carrying amount of goodwill were as follows:
 
 
December 31,
 
 
2012
 
2011
(millions)
 
 
 
 
Goodwill, Beginning Balance
 
$
696

 
$
696

Amount Allocated to Sale of Business (1)
 
(61
)
 
—

Goodwill, Ending Balance
 
$
635

 
$
696

(1) See Note 3.  Acquisitions and Divestitures.