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Accounts Receivable (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 12 Months Ended
Mar. 31, 2012
Dec. 31, 2011
Accounts Receivable    
Net accounts receivable, maximum age of balances past their scheduled invoice date (in days) 30 days 30 days
Allowance for doubtful accounts receivable $ 5 $ 5
Advance cash payments received from third parties to mitigate credit risk $ 177 $ 186