Investments (Summary of Amount of Gross Unrealized Losses for Debt Securities and Estimated Fair Value) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Total investment securities, gross unrealized losses, less than 12 months	$ 21	$ 7
Total investment securities, estimated fair value, less than 12 months	1,625	1,690
Total investment securities, gross unrealized losses, 12 months or more	36	80
Total investment securities, estimated fair value, 12 months or more	1,261	4,839
Total investment securities, gross unrealized losses, total	57	87
Total investment securities, estimated fair value, total	2,886	6,529
Municipal securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Held-to-maturity, Less than 12 months - Gross unrealized losses	1	0
Available-for-sale, Less than 12 months - Gross unrealized losses	0	0
Held-to-maturity, Less than 12 months - Estimated fair value	96	73
Available-for-sale, estimated fair value, less than 12 Months	19	50
Held-to-maturity, 12 months or more - Gross unrealized losses	0	0
Available-for-sale, 12 months or more - Gross unrealized losses	0	0
Held-to-maturity, 12 months or more - Estimated fair value	12	45
Available-for-sale, estimated fair value, 12 months or more	0	5
Held-to-maturity, Total - Gross unrealized losses	1	0
Available-for-sale, Total - Gross unrealized losses	0	0
Held-to-maturity, Total - Estimated fair value	108	118
Available-for-sale, estimated fair value, total	19	55
U.S. Treasury securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Available-for-sale, Less than 12 months - Gross unrealized losses	13
Available-for-sale, estimated fair value, less than 12 Months	142
Available-for-sale, 12 months or more - Gross unrealized losses	0
Available-for-sale, estimated fair value, 12 months or more	0
Available-for-sale, Total - Gross unrealized losses	13
Available-for-sale, estimated fair value, total	142
Agency securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Available-for-sale, Less than 12 months - Gross unrealized losses	0	2
Available-for-sale, estimated fair value, less than 12 Months	6	222
Available-for-sale, 12 months or more - Gross unrealized losses	0	2
Available-for-sale, estimated fair value, 12 months or more	2	359
Available-for-sale, Total - Gross unrealized losses	0	4
Available-for-sale, estimated fair value, total	8	581
Agency guaranteed mortgage-backed securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Available-for-sale, Less than 12 months - Gross unrealized losses	7	4
Available-for-sale, estimated fair value, less than 12 Months	1,197	1,173
Available-for-sale, 12 months or more - Gross unrealized losses	1	38
Available-for-sale, estimated fair value, 12 months or more	179	3,215
Available-for-sale, Total - Gross unrealized losses	8	42
Available-for-sale, estimated fair value, total	1,376	4,388
Small Business Administration loan-backed securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Available-for-sale, Less than 12 months - Gross unrealized losses	0	1
Available-for-sale, estimated fair value, less than 12 Months	15	172
Available-for-sale, 12 months or more - Gross unrealized losses	35	40
Available-for-sale, estimated fair value, 12 months or more	1,068	1,215
Available-for-sale, Total - Gross unrealized losses	35	41
Available-for-sale, estimated fair value, total	1,083	1,387
Other
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Available-for-sale, Less than 12 months - Gross unrealized losses	0	0
Available-for-sale, estimated fair value, less than 12 Months	150	0
Available-for-sale, 12 months or more - Gross unrealized losses	0	0
Available-for-sale, estimated fair value, 12 months or more	0	0
Available-for-sale, Total - Gross unrealized losses	0	0
Available-for-sale, estimated fair value, total	150	0
Available-for-sale
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Available-for-sale, Less than 12 months - Gross unrealized losses	20	7
Available-for-sale, estimated fair value, less than 12 Months	1,529	1,617
Available-for-sale, 12 months or more - Gross unrealized losses	36	80
Available-for-sale, estimated fair value, 12 months or more	1,249	4,794
Available-for-sale, Total - Gross unrealized losses	56	87
Available-for-sale, estimated fair value, total	$ 2,778	$ 6,411

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