Reserves for Losses (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Changes in the Card Member loans reserve for losses
Balance, January 1	$ 1,706	$ 1,223
Card Member loans - provisions	499	337
Card Member loans net write-offs - principal	(358)	(272)
Card Member loans net write-offs - interest and fees	(71)	(51)
Card Member loans reserves for losses - other	10	11
Balance, September 30	$ 1,786	$ 1,248