Accounts Receivable (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Accounts Receivable
Advance cash payments received from third parties to mitigate credit risk	$ 152	$ 173
Standby letters of credit	448	343
Net accounts receivable, maximum age of balances past their scheduled invoice date	30 days	30 days
Allowance for doubtful accounts receivable	$ 4	$ 4