Investment Securities Investment Securities (Tax and Nontaxable Income by Investment Type) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Investment [Line Items]
Interest on securities	$ 350	$ 326	$ 204
Other Trading [Member]
Investment [Line Items]
Interest on securities	5	2	3
Available-for-sale Securities [Member]
Investment [Line Items]
Interest on securities	321	301	178
Held-to-maturity Securities [Member]
Investment [Line Items]
Interest on securities	24	23	23
Taxable [Member]
Investment [Line Items]
Interest on securities	310	289	179
Taxable [Member] | Other Trading [Member]
Investment [Line Items]
Interest on securities	5	2	3
Taxable [Member] | Available-for-sale Securities [Member]
Investment [Line Items]
Interest on securities	295	277	166
Taxable [Member] | Held-to-maturity Securities [Member]
Investment [Line Items]
Interest on securities	10	10	10
Nontaxable [Member]
Investment [Line Items]
Interest on securities	40	37	25
Nontaxable [Member] | Other Trading [Member]
Investment [Line Items]
Interest on securities	0	0	0
Nontaxable [Member] | Available-for-sale Securities [Member]
Investment [Line Items]
Interest on securities	26	24	12
Nontaxable [Member] | Held-to-maturity Securities [Member]
Investment [Line Items]
Interest on securities	$ 14	$ 13	$ 13