Investment Securities, Gross Unrealized Losses and Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Available-for-sale securities:
Less than 12 months, Gross unrealized losses, available-for-sale	$ (423)	$ (3,860)
Less than 12 months, Fair value, available-for-sale	63,946	154,806
12 months or more, Gross unrealized losses, available-for-sale	(1,522)	(1,547)
12 months or more, Fair value, available-for-sale	45,731	27,362
Total, Gross unrealized losses, available-for-sale	(1,945)	(5,407)
Total, Fair value, available-for-sale	109,677	182,168
Held-to-maturity securities:
Less than 12 months, Gross unrealized losses, held-to-maturity	(520)	(1,025)
Less than 12 months, Fair value, held-to-maturity	58,489	51,317
12 months or more, Gross unrealized losses, held-to-maturity	(70)	(2)
12 months or more, Fair value, held-to-maturity	5,997	899
Total, Gross unrealized losses, held-to-maturity	(590)	(1,027)
Total, Fair value, held-to-maturity	64,486	52,216
Total investment securities:
Less than 12 months, Gross unrealized losses, investment securities	(943)	(4,885)
Less than 12 months, Fair value, investment securities	122,435	206,123
12 months or more, Gross unrealized losses, investment securities	(1,592)	(1,549)
12 months or more, Fair value, investment securities	51,728	28,261
Total, Gross unrealized losses, investment securities	(2,535)	(6,434)
Total, Fair value, investment securities	174,163	234,384
Total debt securities [Member]
Available-for-sale securities:
Less than 12 months, Gross unrealized losses, available-for-sale	(420)	(3,857)
Less than 12 months, Fair value, available-for-sale	63,915	154,765
12 months or more, Gross unrealized losses, available-for-sale	(1,518)	(1,539)
12 months or more, Fair value, available-for-sale	45,664	27,317
Total, Gross unrealized losses, available-for-sale	(1,938)	(5,396)
Total, Fair value, available-for-sale	109,579	182,082
Securities of U.S. Treasury and federal agencies [Member]
Available-for-sale securities:
Less than 12 months, Gross unrealized losses, available-for-sale	(4)	(109)
Less than 12 months, Fair value, available-for-sale	2,582	10,816
12 months or more, Gross unrealized losses, available-for-sale	(62)	0
12 months or more, Fair value, available-for-sale	1,968	0
Total, Gross unrealized losses, available-for-sale	(66)	(109)
Total, Fair value, available-for-sale	4,550	10,816
Held-to-maturity securities:
Less than 12 months, Gross unrealized losses, held-to-maturity	(36)	(77)
Less than 12 months, Fair value, held-to-maturity	3,345	6,351
12 months or more, Gross unrealized losses, held-to-maturity	0	0
12 months or more, Fair value, held-to-maturity	0	0
Total, Gross unrealized losses, held-to-maturity	(36)	(77)
Total, Fair value, held-to-maturity	3,345	6,351
Securities of U.S. states and political subdivisions [Member]
Available-for-sale securities:
Less than 12 months, Gross unrealized losses, available-for-sale	(23)	(341)
Less than 12 months, Fair value, available-for-sale	6,117	17,412
12 months or more, Gross unrealized losses, available-for-sale	(831)	(1,230)
12 months or more, Fair value, available-for-sale	19,188	16,213
Total, Gross unrealized losses, available-for-sale	(854)	(1,571)
Total, Fair value, available-for-sale	25,305	33,625
Held-to-maturity securities:
Less than 12 months, Gross unrealized losses, held-to-maturity	(19)	(144)
Less than 12 months, Fair value, held-to-maturity	2,016	4,871
12 months or more, Gross unrealized losses, held-to-maturity	(26)	0
12 months or more, Fair value, held-to-maturity	785	0
Total, Gross unrealized losses, held-to-maturity	(45)	(144)
Total, Fair value, held-to-maturity	2,801	4,871
Mortgage-backed securities: [Member]
Available-for-sale securities:
Less than 12 months, Gross unrealized losses, available-for-sale	(386)	(3,385)
Less than 12 months, Fair value, available-for-sale	51,246	122,721
12 months or more, Gross unrealized losses, available-for-sale	(553)	(155)
12 months or more, Fair value, available-for-sale	22,515	5,416
Total, Gross unrealized losses, available-for-sale	(939)	(3,540)
Total, Fair value, available-for-sale	73,761	128,137
Federal agencies [Member]
Available-for-sale securities:
Less than 12 months, Gross unrealized losses, available-for-sale	(383)	(3,338)
Less than 12 months, Fair value, available-for-sale	50,708	120,735
12 months or more, Gross unrealized losses, available-for-sale	(545)	(120)
12 months or more, Fair value, available-for-sale	22,103	3,481
Total, Gross unrealized losses, available-for-sale	(928)	(3,458)
Total, Fair value, available-for-sale	72,811	124,216
Held-to-maturity securities:
Less than 12 months, Gross unrealized losses, held-to-maturity	(465)	(804)
Less than 12 months, Fair value, held-to-maturity	53,128	40,095
12 months or more, Gross unrealized losses, held-to-maturity	(44)	0
12 months or more, Fair value, held-to-maturity	5,212	0
Total, Gross unrealized losses, held-to-maturity	(509)	(804)
Total, Fair value, held-to-maturity	58,340	40,095
Residential [Member]
Available-for-sale securities:
Less than 12 months, Gross unrealized losses, available-for-sale	(2)	(4)
Less than 12 months, Fair value, available-for-sale	145	527
12 months or more, Gross unrealized losses, available-for-sale	(1)	(4)
12 months or more, Fair value, available-for-sale	64	245
Total, Gross unrealized losses, available-for-sale	(3)	(8)
Total, Fair value, available-for-sale	209	772
Commercial [Member]
Available-for-sale securities:
Less than 12 months, Gross unrealized losses, available-for-sale	(1)	(43)
Less than 12 months, Fair value, available-for-sale	393	1,459
12 months or more, Gross unrealized losses, available-for-sale	(7)	(31)
12 months or more, Fair value, available-for-sale	348	1,690
Total, Gross unrealized losses, available-for-sale	(8)	(74)
Total, Fair value, available-for-sale	741	3,149
Corporate debt securities [Member]
Available-for-sale securities:
Less than 12 months, Gross unrealized losses, available-for-sale	(5)	(11)
Less than 12 months, Fair value, available-for-sale	305	946
12 months or more, Gross unrealized losses, available-for-sale	(60)	(99)
12 months or more, Fair value, available-for-sale	886	1,229
Total, Gross unrealized losses, available-for-sale	(65)	(110)
Total, Fair value, available-for-sale	1,191	2,175
Collateralized loan and other debt obligations [Member]
Available-for-sale securities:
Less than 12 months, Gross unrealized losses, available-for-sale	(1)	(2)
Less than 12 months, Fair value, available-for-sale	3,171	1,899
12 months or more, Gross unrealized losses, available-for-sale	(6)	(29)
12 months or more, Fair value, available-for-sale	581	3,197
Total, Gross unrealized losses, available-for-sale	(7)	(31)
Total, Fair value, available-for-sale	3,752	5,096
Held-to-maturity securities:
Less than 12 months, Gross unrealized losses, held-to-maturity	0	0
Less than 12 months, Fair value, held-to-maturity	0	0
12 months or more, Gross unrealized losses, held-to-maturity	0	(1)
12 months or more, Fair value, held-to-maturity	0	266
Total, Gross unrealized losses, held-to-maturity	0	(1)
Total, Fair value, held-to-maturity	0	266
Other [Member]
Available-for-sale securities:
Less than 12 months, Gross unrealized losses, available-for-sale	(1)	(9)
Less than 12 months, Fair value, available-for-sale	494	971
12 months or more, Gross unrealized losses, available-for-sale	(6)	(26)
12 months or more, Fair value, available-for-sale	526	1,262
Total, Gross unrealized losses, available-for-sale	(7)	(35)
Total, Fair value, available-for-sale	1,020	2,233
Held-to-maturity securities:
Less than 12 months, Gross unrealized losses, held-to-maturity	0	0
Less than 12 months, Fair value, held-to-maturity	0	0
12 months or more, Gross unrealized losses, held-to-maturity	0	(1)
12 months or more, Fair value, held-to-maturity	0	633
Total, Gross unrealized losses, held-to-maturity	0	(1)
Total, Fair value, held-to-maturity	0	633
Total marketable equity securities [Member]
Available-for-sale securities:
Less than 12 months, Gross unrealized losses, available-for-sale	(3)	(3)
Less than 12 months, Fair value, available-for-sale	31	41
12 months or more, Gross unrealized losses, available-for-sale	(4)	(8)
12 months or more, Fair value, available-for-sale	67	45
Total, Gross unrealized losses, available-for-sale	(7)	(11)
Total, Fair value, available-for-sale	98	86
Perpetual preferred securities [Member]
Available-for-sale securities:
Less than 12 months, Gross unrealized losses, available-for-sale	(1)	(3)
Less than 12 months, Fair value, available-for-sale	21	41
12 months or more, Gross unrealized losses, available-for-sale	(4)	(8)
12 months or more, Fair value, available-for-sale	67	45
Total, Gross unrealized losses, available-for-sale	(5)	(11)
Total, Fair value, available-for-sale	88	86
Other marketable equity securities [Member]
Available-for-sale securities:
Less than 12 months, Gross unrealized losses, available-for-sale	(2)	0
Less than 12 months, Fair value, available-for-sale	10	0
12 months or more, Gross unrealized losses, available-for-sale	0	0
12 months or more, Fair value, available-for-sale	0	0
Total, Gross unrealized losses, available-for-sale	(2)	0
Total, Fair value, available-for-sale	$ 10	$ 0