Business Segments and Revenue from Contracts with Customers - Revenue from Contracts with Customers (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation of Revenue [Line Items]
Total contract revenue	$ 1,241	$ 805	$ 2,242	$ 1,742
Revenue (reduction) from other sources	390	291	680	578
Non-interest income (loss)	1,631	1,096	2,922	2,320
Operating segments | Credit Card
Disaggregation of Revenue [Line Items]
Total contract revenue	1,019	669	1,832	1,427
Revenue (reduction) from other sources	234	176	450	329
Non-interest income (loss)	1,253	845	2,282	1,756
Operating segments | Consumer Banking
Disaggregation of Revenue [Line Items]
Total contract revenue	131	82	251	215
Revenue (reduction) from other sources	13	15	34	8
Non-interest income (loss)	144	97	285	223
Operating segments | Commercial Banking
Disaggregation of Revenue [Line Items]
Total contract revenue	91	56	159	103
Revenue (reduction) from other sources	166	124	338	315
Non-interest income (loss)	257	180	497	418
Other
Disaggregation of Revenue [Line Items]
Total contract revenue	0	(2)	0	(3)
Revenue (reduction) from other sources	(23)	(24)	(142)	(74)
Non-interest income (loss)	(23)	(26)	(142)	(77)
Interchange fees, net
Disaggregation of Revenue [Line Items]
Total contract revenue	1,016	672	1,833	1,424
Interchange fees, net | Operating segments | Credit Card
Disaggregation of Revenue [Line Items]
Total contract revenue	925	612	1,668	1,300
Interchange fees, net | Operating segments | Consumer Banking
Disaggregation of Revenue [Line Items]
Total contract revenue	67	48	124	98
Interchange fees, net | Operating segments | Commercial Banking
Disaggregation of Revenue [Line Items]
Total contract revenue	24	13	41	28
Interchange fees, net | Other
Disaggregation of Revenue [Line Items]
Total contract revenue	0	(1)	0	(2)
Service charges and other customer-related fees
Disaggregation of Revenue [Line Items]
Total contract revenue	111	73	205	168
Service charges and other customer-related fees | Operating segments | Credit Card
Disaggregation of Revenue [Line Items]
Total contract revenue	0	0	0	0
Service charges and other customer-related fees | Operating segments | Consumer Banking
Disaggregation of Revenue [Line Items]
Total contract revenue	45	31	89	95
Service charges and other customer-related fees | Operating segments | Commercial Banking
Disaggregation of Revenue [Line Items]
Total contract revenue	66	43	116	74
Service charges and other customer-related fees | Other
Disaggregation of Revenue [Line Items]
Total contract revenue	0	(1)	0	(1)
Other
Disaggregation of Revenue [Line Items]
Total contract revenue	114	60	204	150
Other | Operating segments | Credit Card
Disaggregation of Revenue [Line Items]
Total contract revenue	94	57	164	127
Other | Operating segments | Consumer Banking
Disaggregation of Revenue [Line Items]
Total contract revenue	19	3	38	22
Other | Operating segments | Commercial Banking
Disaggregation of Revenue [Line Items]
Total contract revenue	1	0	2	1
Other | Other
Disaggregation of Revenue [Line Items]
Total contract revenue	$ 0	$ 0	$ 0	$ 0