Investments (Summary of Amount of Gross Unrealized Losses for Debt Securities and Estimated Fair Value) (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Gross unrealized losses
Total investment securities, less than 12 months	$ 162	$ 21
Total investment securities, 12 months or more	77	36
Total investment securities, total	239	57
Estimated fair value
Total investment securities, less than 12 months	11,042	1,625
Total investment securities, 12 months or more	1,487	1,261
Total investment securities, total	12,529	2,886
Municipal securities
Gross unrealized losses
Held-to-maturity, less than 12 Months	2	1
Available-for-sale, less than 12 Months	5	0
Held-to-maturity, 12 months or more	0	0
Available-for-sale, 12 Months or more	0	0
Held-to-maturity, total	2	1
Available-for-sale, total	5	0
Estimated fair value
Held-to-maturity, less than 12 Months	121	96
Available-for-sale, less than 12 Months	377	19
Held-to-maturity, 12 months or more	13	12
Available-for-sale, 12 months or more	2	0
Held-to-maturity, total	134	108
Available-for-sale, total	379	19
U.S. Treasury, agencies and corporations
Gross unrealized losses
Available-for-sale, less than 12 Months	0	13
Available-for-sale, 12 Months or more	27	0
Available-for-sale, total	27	13
Estimated fair value
Available-for-sale, less than 12 Months	0	142
Available-for-sale, 12 months or more	128	0
Available-for-sale, total	128	142
Agency securities
Gross unrealized losses
Available-for-sale, less than 12 Months	0	0
Available-for-sale, 12 Months or more	0	0
Available-for-sale, total	0	0
Estimated fair value
Available-for-sale, less than 12 Months	110	6
Available-for-sale, 12 months or more	1	2
Available-for-sale, total	111	8
Agency guaranteed mortgage-backed securities
Gross unrealized losses
Available-for-sale, less than 12 Months	155	7
Available-for-sale, 12 Months or more	20	1
Available-for-sale, total	175	8
Estimated fair value
Available-for-sale, less than 12 Months	10,434	1,197
Available-for-sale, 12 months or more	534	179
Available-for-sale, total	10,968	1,376
Small Business Administration loan-backed securities
Gross unrealized losses
Available-for-sale, less than 12 Months	0	0
Available-for-sale, 12 Months or more	30	35
Available-for-sale, total	30	35
Estimated fair value
Available-for-sale, less than 12 Months	0	15
Available-for-sale, 12 months or more	809	1,068
Available-for-sale, total	809	1,083
Other
Gross unrealized losses
Available-for-sale, less than 12 Months	0	0
Available-for-sale, 12 Months or more	0	0
Available-for-sale, total	0	0
Estimated fair value
Available-for-sale, less than 12 Months	0	150
Available-for-sale, 12 months or more	0	0
Available-for-sale, total	0	150
Available-for-sale
Gross unrealized losses
Available-for-sale, less than 12 Months	160	20
Available-for-sale, 12 Months or more	77	36
Available-for-sale, total	237	56
Estimated fair value
Available-for-sale, less than 12 Months	10,921	1,529
Available-for-sale, 12 months or more	1,474	1,249
Available-for-sale, total	$ 12,395	$ 2,778