Summary of Significant Accounting Policies (Details) - USD ($) $ in Millions	3 Months Ended				12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues
Other					$ 1,360	$ 1,457	$ 1,678
Expenses
Marketing and promotion					6,470	5,722	6,249
Card Member rewards					9,696	8,687	7,819
Discount Revenue
Revenues
Revenues and Fees					$ 24,721	22,890	22,377
Revenue Recognition Standard Impacts [Member]
Revenues
Other	$ (78)	$ (71)	$ (64)	$ (65)		(278)	(253)
Expenses
Marketing and promotion	617	591	593	549		2,350	2,420
Card Member rewards	286	268	271	254		1,079	1,026
Revenue Recognition Standard Impacts [Member] | Discount Revenue
Revenues
Revenues and Fees	$ 981	$ 930	$ 928	$ 868		$ 3,707	$ 3,699