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Summary of Significant Accounting Policies (Details) (USD $)
In Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2011
Jun. 30, 2010
Jun. 30, 2011
Jun. 30, 2010
Accounts Receivable (Numeric)        
Purchases of accounts receivable $ 187 $ 149 $ 452 $ 225