Investment Securities, Unrealized Loss Position, by Credit Rating (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Available-for-sale securities:
Gross unrealized losses	$ (1,945)	$ (5,407)
Fair value	272,210	308,364
Held-to-maturity securities:
Gross unrealized losses	(590)	(1,027)
Fair value	142,818	99,155
Total investment securities:
Gross unrealized losses	(2,535)	(6,434)
Fair value	415,028	407,519
Total debt securities [Member]
Available-for-sale securities:
Gross unrealized losses	(1,938)	(5,396)
Fair value	271,317	307,153
Held-to-maturity securities:
Fair value	142,818	99,155
Securities of U.S. Treasury and federal agencies [Member]
Available-for-sale securities:
Gross unrealized losses	(66)	(109)
Fair value	6,350	25,819
Held-to-maturity securities:
Gross unrealized losses	(36)	(77)
Fair value	45,282	45,079
Securities of U.S. states and political subdivisions [Member]
Available-for-sale securities:
Gross unrealized losses	(854)	(1,571)
Fair value	52,774	51,101
Held-to-maturity securities:
Gross unrealized losses	(45)	(144)
Fair value	6,346	6,209
Mortgage-backed securities: [Member]
Available-for-sale securities:
Gross unrealized losses	(939)	(3,540)
Fair value	161,227	177,548
Federal agencies [Member]
Available-for-sale securities:
Gross unrealized losses	(928)	(3,458)
Fair value	150,181	161,230
Held-to-maturity securities:
Gross unrealized losses	(509)	(804)
Fair value	89,867	44,457
Residential [Member]
Available-for-sale securities:
Gross unrealized losses	(3)	(8)
Fair value	6,394	7,816
Commercial [Member]
Available-for-sale securities:
Gross unrealized losses	(8)	(74)
Fair value	4,652	8,502
Corporate debt securities [Member]
Available-for-sale securities:
Gross unrealized losses	(65)	(110)
Fair value	9,340	11,457
Collateralized loan and other debt obligations [Member]
Available-for-sale securities:
Gross unrealized losses	(7)	(31)
Fair value	35,608	35,020
Held-to-maturity securities:
Gross unrealized losses	0	(1)
Fair value	664	1,070
Other [Member]
Available-for-sale securities:
Gross unrealized losses	(7)	(35)
Fair value	6,018	6,208
Held-to-maturity securities:
Gross unrealized losses	0	(1)
Fair value	659	2,340
Total marketable equity securities [Member]
Available-for-sale securities:
Gross unrealized losses	(7)	(11)
Fair value	893	1,211
Perpetual preferred securities [Member]
Available-for-sale securities:
Gross unrealized losses	(5)	(11)
Fair value	419	469
Unrated Investment Grade Securities [Member]
Total investment securities:
Gross unrealized losses	(27)	(54)
Fair value	5,700	7,000
Investment grade [Member]
Available-for-sale securities:
Gross unrealized losses	(1,849)	(5,202)
Fair value	108,397	179,308
Held-to-maturity securities:
Gross unrealized losses	(589)	(1,026)
Fair value	64,394	52,199
Total investment securities:
Gross unrealized losses	(2,438)	(6,228)
Fair value	172,791	231,507
Investment grade [Member] | Total debt securities [Member]
Available-for-sale securities:
Gross unrealized losses	(1,845)	(5,192)
Fair value	108,327	179,240
Investment grade [Member] | Securities of U.S. Treasury and federal agencies [Member]
Available-for-sale securities:
Gross unrealized losses	(66)	(109)
Fair value	4,550	10,816
Held-to-maturity securities:
Gross unrealized losses	(36)	(77)
Fair value	3,345	6,351
Investment grade [Member] | Securities of U.S. states and political subdivisions [Member]
Available-for-sale securities:
Gross unrealized losses	(822)	(1,517)
Fair value	25,098	33,271
Held-to-maturity securities:
Gross unrealized losses	(45)	(144)
Fair value	2,801	4,871
Investment grade [Member] | Mortgage-backed securities: [Member]
Available-for-sale securities:
Gross unrealized losses	(931)	(3,474)
Fair value	73,472	126,977
Investment grade [Member] | Federal agencies [Member]
Available-for-sale securities:
Gross unrealized losses	(928)	(3,458)
Fair value	72,811	124,216
Held-to-maturity securities:
Gross unrealized losses	(508)	(803)
Fair value	58,248	40,078
Investment grade [Member] | Residential [Member]
Available-for-sale securities:
Gross unrealized losses	(1)	(1)
Fair value	134	176
Investment grade [Member] | Commercial [Member]
Available-for-sale securities:
Gross unrealized losses	(2)	(15)
Fair value	527	2,585
Investment grade [Member] | Corporate debt securities [Member]
Available-for-sale securities:
Gross unrealized losses	(14)	(31)
Fair value	674	1,238
Investment grade [Member] | Collateralized loan and other debt obligations [Member]
Available-for-sale securities:
Gross unrealized losses	(7)	(31)
Fair value	3,752	5,096
Held-to-maturity securities:
Gross unrealized losses	0	(1)
Fair value	0	266
Investment grade [Member] | Other [Member]
Available-for-sale securities:
Gross unrealized losses	(5)	(30)
Fair value	781	1,842
Held-to-maturity securities:
Gross unrealized losses	0	(1)
Fair value	0	633
Investment grade [Member] | Perpetual preferred securities [Member]
Available-for-sale securities:
Gross unrealized losses	(4)	(10)
Fair value	70	68
Non-investment grade [Member]
Available-for-sale securities:
Gross unrealized losses	(94)	(205)
Fair value	1,270	2,860
Held-to-maturity securities:
Gross unrealized losses	(1)	(1)
Fair value	92	17
Total investment securities:
Gross unrealized losses	(95)	(206)
Fair value	1,362	2,877
Non-investment grade [Member] | Total debt securities [Member]
Available-for-sale securities:
Gross unrealized losses	(93)	(204)
Fair value	1,252	2,842
Non-investment grade [Member] | Securities of U.S. Treasury and federal agencies [Member]
Available-for-sale securities:
Gross unrealized losses	0	0
Fair value	0	0
Held-to-maturity securities:
Gross unrealized losses	0	0
Fair value	0	0
Non-investment grade [Member] | Securities of U.S. states and political subdivisions [Member]
Available-for-sale securities:
Gross unrealized losses	(32)	(54)
Fair value	207	354
Held-to-maturity securities:
Gross unrealized losses	0	0
Fair value	0	0
Non-investment grade [Member] | Mortgage-backed securities: [Member]
Available-for-sale securities:
Gross unrealized losses	(8)	(66)
Fair value	289	1,160
Non-investment grade [Member] | Federal agencies [Member]
Available-for-sale securities:
Gross unrealized losses	0	0
Fair value	0	0
Held-to-maturity securities:
Gross unrealized losses	(1)	(1)
Fair value	92	17
Non-investment grade [Member] | Residential [Member]
Available-for-sale securities:
Gross unrealized losses	(2)	(7)
Fair value	75	596
Non-investment grade [Member] | Commercial [Member]
Available-for-sale securities:
Gross unrealized losses	(6)	(59)
Fair value	214	564
Non-investment grade [Member] | Corporate debt securities [Member]
Available-for-sale securities:
Gross unrealized losses	(51)	(79)
Fair value	517	937
Non-investment grade [Member] | Collateralized loan and other debt obligations [Member]
Available-for-sale securities:
Gross unrealized losses	0	0
Fair value	0	0
Held-to-maturity securities:
Gross unrealized losses	0	0
Fair value	0	0
Non-investment grade [Member] | Other [Member]
Available-for-sale securities:
Gross unrealized losses	(2)	(5)
Fair value	239	391
Held-to-maturity securities:
Gross unrealized losses	0	0
Fair value	0	0
Non-investment grade [Member] | Perpetual preferred securities [Member]
Available-for-sale securities:
Gross unrealized losses	(1)	(1)
Fair value	$ 18	$ 18