Basis of Presentation (Details) - USD ($) $ in Millions	3 Months Ended					12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Revenues
Discount revenue	$ 5,889				$ 5,387
Other	377				361
Expenses
Marketing and business development	1,345				1,285
Card Member Rewards	$ 2,347				2,061
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