Investment Securities (Tax and Nontaxable Income by Investment Type) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Investment [Line Items]
Interest on securities	$ 362	$ 350	$ 326
Other Trading [Member]
Investment [Line Items]
Interest on securities	7	5	2
Available-for-sale Securities [Member]
Investment [Line Items]
Interest on securities	333	321	301
Held-to-maturity Securities [Member]
Investment [Line Items]
Interest on securities	22	24	23
Taxable [Member]
Investment [Line Items]
Interest on securities	318	306	287
Taxable [Member] | Other Trading [Member]
Investment [Line Items]
Interest on securities	1	1	0
Taxable [Member] | Available-for-sale Securities [Member]
Investment [Line Items]
Interest on securities	308	295	277
Taxable [Member] | Held-to-maturity Securities [Member]
Investment [Line Items]
Interest on securities	9	10	10
Nontaxable [Member]
Investment [Line Items]
Interest on securities	44	44	39
Nontaxable [Member] | Other Trading [Member]
Investment [Line Items]
Interest on securities	6	4	2
Nontaxable [Member] | Available-for-sale Securities [Member]
Investment [Line Items]
Interest on securities	25	26	24
Nontaxable [Member] | Held-to-maturity Securities [Member]
Investment [Line Items]
Interest on securities	$ 13	$ 14	$ 13