Customer Deposits (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
U.S.:
Interest-bearing	$ 65,913	$ 63,666
Non-interest-bearing	353	395
Non-U.S.:
Interest-bearing	42	34
Non-interest-bearing	357	357
Total customer deposits	66,665	64,452
Card Member Credit Balances [Member]
U.S.:
Non-interest-bearing	319	358
Non-U.S.:
Non-interest-bearing	$ 344	$ 344