Investment Securities Investment Securities (Tax and Nontaxable Income by Investment Type) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investment [Line Items]
Interest on securities	$ 326	$ 204	$ 124
Trading Securities [Member]
Investment [Line Items]
Interest on securities	2	3	2
Available-for-sale Securities [Member]
Investment [Line Items]
Interest on securities	301	178	98
Held-to-maturity Securities [Member]
Investment [Line Items]
Interest on securities	23	23	24
Taxable [Member]
Investment [Line Items]
Interest on securities	289	179	110
Taxable [Member] | Trading Securities [Member]
Investment [Line Items]
Interest on securities	2	3	2
Taxable [Member] | Available-for-sale Securities [Member]
Investment [Line Items]
Interest on securities	277	166	95
Taxable [Member] | Held-to-maturity Securities [Member]
Investment [Line Items]
Interest on securities	10	10	13
Nontaxable [Member]
Investment [Line Items]
Interest on securities	37	25	14
Nontaxable [Member] | Trading Securities [Member]
Investment [Line Items]
Interest on securities	0	0	0
Nontaxable [Member] | Available-for-sale Securities [Member]
Investment [Line Items]
Interest on securities	24	12	3
Nontaxable [Member] | Held-to-maturity Securities [Member]
Investment [Line Items]
Interest on securities	$ 13	$ 13	$ 11