American Express Relationship - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2015
Deferred revenue, additions	$ 675,000,000	$ 675,000,000	$ 675,000,000	$ 675,000,000
Fuel card obligation
Line of credit facility, maximum borrowing capacity					$ 612,000,000
Fuel card obligation	$ 561,000,000				$ 221,000,000