Basis of Presentation (Details) - USD ($) $ in Millions	3 Months Ended					6 Months Ended		12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Revenues
Discount revenue	$ 6,194			$ 5,743		$ 12,083	$ 11,130
Other	349			379		726	740
Expenses
Marketing and business development	1,663			1,456		3,008	2,741
Card Member rewards	$ 2,433			2,196		$ 4,780	$ 4,257
Revenue Recognition Standard Impacts [Member]
Revenues
Discount revenue		$ 981	$ 930	928	$ 868			$ 3,707	$ 3,699
Other		(78)	(71)	(64)	(65)			(278)	(253)
Expenses
Marketing and business development		617	591	593	549			2,350	2,420
Card Member rewards		$ 286	$ 268	$ 271	$ 254			$ 1,079	$ 1,026