Accounts Receivable (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2014	Dec. 31, 2013
Accounts Receivable
Advance cash payments received from third parties to mitigate credit risk	$ 105	$ 117
Standby letters of credit	206	426
Net accounts receivable, maximum age of balances past their scheduled invoice date	30 days	30 days
Allowance for doubtful accounts receivable	$ 4	$ 5