Customer Deposits (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
U.S.:
Interest-bearing	$ 68,572	$ 63,666
Non-interest-bearing	355	395
Non-U.S.:
Interest-bearing	28	34
Non-interest-bearing	346	357
Total customer deposits	69,301	64,452
Card Member Credit Balances [Member]
U.S.:
Non-interest-bearing	321	358
Non-U.S.:
Non-interest-bearing	$ 336	$ 344