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Allowance for Doubtful Accounts (Tables)
 12 Months Ended
Dec. 31, 2012
Allowance for Doubtful Accounts [Abstract]  
Changes in the allowance for doubtful accounts
Changes in the allowance for doubtful accounts were as follows:
 
 
Year Ended December 31,
 
 
2012
 
2011
 
2010
(millions)
 
 
 
 
 
 
Balance, Beginning of Period
 
$
9

 
$
27

 
$
31

Changes
 
 

 
 

 
 

Changes in Ecuador Receivable, Net(1)
 
—

 
(19
)
 
(6
)
Other Changes
 
1

 
1

 
2

Net Changes
 
1

 
(18
)
 
(4
)
Balance, End of Period
 
$
10

 
$
9

 
$
27

 
(1) 
During 2011, recovery of approximately $19 million for outstanding receivables was included in the final terms of our agreement to transfer our assets and the associated electricity concession and PSC to the Ecuadorian government. See Note 3.  Acquisitions and Divestitures.