Basis of Presentation (Details) - USD ($) $ in Millions	3 Months Ended					9 Months Ended		12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Revenues
Discount revenue	$ 6,181		$ 5,700			$ 18,264	$ 16,830
Other	334		372			1,060	1,112
Expenses
Marketing and business development	1,642		1,446			4,650	4,187
Card Member rewards	$ 2,400		2,168			$ 7,180	$ 6,425
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