Accounts Receivable (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2012	Dec. 31, 2011
Accounts Receivable
Net accounts receivable, maximum age of balances past their scheduled invoice date	30 days	30 days
Allowance for doubtful accounts receivable	$ 4	$ 5
Advance cash payments received from third parties to mitigate credit risk	$ 216	$ 186