Investment Securities, Major Categories (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Available-for-sale securities:
Amortized Cost	$ 270,385	$ 310,153
Gross unrealized gains	3,770	3,618
Gross unrealized losses	(1,945)	(5,407)
Fair value	272,210	308,364
Held-to-maturity securities:
Held-to-maturity securities, Amortized Cost	142,423	99,583
Gross unrealized gains	985	599
Gross unrealized losses	(590)	(1,027)
Held-to-maturity, at fair value	142,818	99,155
Total investment securities:
Amortized Cost	412,808	409,736
Gross unrealized gains	4,755	4,217
Gross unrealized losses	(2,535)	(6,434)
Fair value	415,028	407,519
Held-to-maturity, at fair value	142,818	99,155
Collateralized Debt Obligations [Member]
Available-for-sale securities:
Amortized Cost	914	819
Fair value	1,000	847
Asset-backed Securities, Securitized Loans and Receivables [Member]
Held-to-maturity securities:
Held-to-maturity securities, Amortized Cost	158	1,300
Held-to-maturity, at fair value	158	1,300
Total investment securities:
Held-to-maturity, at fair value	158	1,300
Asset-backed Securities Collateralized by Dealer Floorplan Loans [Member]
Held-to-maturity securities:
Held-to-maturity securities, Amortized Cost	500	1,100
Held-to-maturity, at fair value	501	1,100
Total investment securities:
Held-to-maturity, at fair value	501	1,100
Total debt securities [Member]
Available-for-sale securities:
Amortized Cost	269,779	309,447
Gross unrealized gains	3,476	3,102
Gross unrealized losses	(1,938)	(5,396)
Fair value	271,317	307,153
Held-to-maturity securities:
Held-to-maturity securities, Amortized Cost	142,423	99,583
Held-to-maturity, at fair value	142,818	99,155
Total investment securities:
Held-to-maturity, at fair value	142,818	99,155
Securities of U.S. Treasury and federal agencies [Member]
Available-for-sale securities:
Amortized Cost	6,408	25,874
Gross unrealized gains	8	54
Gross unrealized losses	(66)	(109)
Fair value	6,350	25,819
Held-to-maturity securities:
Held-to-maturity securities, Amortized Cost	44,712	44,690
Gross unrealized gains	606	466
Gross unrealized losses	(36)	(77)
Held-to-maturity, at fair value	45,282	45,079
Total investment securities:
Held-to-maturity, at fair value	45,282	45,079
Securities of U.S. states and political subdivisions [Member]
Available-for-sale securities:
Amortized Cost	52,854	52,121
Gross unrealized gains	774	551
Gross unrealized losses	(854)	(1,571)
Fair value	52,774	51,101
Held-to-maturity securities:
Held-to-maturity securities, Amortized Cost	6,321	6,336
Gross unrealized gains	70	17
Gross unrealized losses	(45)	(144)
Held-to-maturity, at fair value	6,346	6,209
Total investment securities:
Held-to-maturity, at fair value	6,346	6,209
Mortgage-backed securities: [Member]
Available-for-sale securities:
Amortized Cost	160,400	179,363
Gross unrealized gains	1,766	1,725
Gross unrealized losses	(939)	(3,540)
Fair value	161,227	177,548
Federal agencies [Member]
Available-for-sale securities:
Amortized Cost	149,872	163,513
Gross unrealized gains	1,237	1,175
Gross unrealized losses	(928)	(3,458)
Fair value	150,181	161,230
Held-to-maturity securities:
Held-to-maturity securities, Amortized Cost	90,071	45,161
Gross unrealized gains	305	100
Gross unrealized losses	(509)	(804)
Held-to-maturity, at fair value	89,867	44,457
Total investment securities:
Held-to-maturity, at fair value	89,867	44,457
Residential [Member]
Available-for-sale securities:
Amortized Cost	5,942	7,375
Gross unrealized gains	455	449
Gross unrealized losses	(3)	(8)
Fair value	6,394	7,816
Commercial [Member]
Available-for-sale securities:
Amortized Cost	4,586	8,475
Gross unrealized gains	74	101
Gross unrealized losses	(8)	(74)
Fair value	4,652	8,502
Corporate debt securities [Member]
Available-for-sale securities:
Amortized Cost	8,962	11,186
Gross unrealized gains	443	381
Gross unrealized losses	(65)	(110)
Fair value	9,340	11,457
Collateralized loan and other debt obligations [Member]
Available-for-sale securities:
Amortized Cost	35,298	34,764
Gross unrealized gains	317	287
Gross unrealized losses	(7)	(31)
Fair value	35,608	35,020
Held-to-maturity securities:
Held-to-maturity securities, Amortized Cost	661	1,065
Gross unrealized gains	3	6
Gross unrealized losses	0	(1)
Held-to-maturity, at fair value	664	1,070
Total investment securities:
Held-to-maturity, at fair value	664	1,070
Other [Member]
Available-for-sale securities:
Amortized Cost	5,857	6,139
Gross unrealized gains	168	104
Gross unrealized losses	(7)	(35)
Fair value	6,018	6,208
Held-to-maturity securities:
Held-to-maturity securities, Amortized Cost	658	2,331
Gross unrealized gains	1	10
Gross unrealized losses	0	(1)
Held-to-maturity, at fair value	659	2,340
Total investment securities:
Held-to-maturity, at fair value	659	2,340
Total marketable equity securities [Member]
Available-for-sale securities:
Amortized Cost	606	706
Gross unrealized gains	294	516
Gross unrealized losses	(7)	(11)
Fair value	893	1,211
Perpetual preferred securities [Member]
Available-for-sale securities:
Amortized Cost	412	445
Gross unrealized gains	12	35
Gross unrealized losses	(5)	(11)
Fair value	419	469
Other marketable equity securities [Member]
Available-for-sale securities:
Amortized Cost	194	261
Gross unrealized gains	282	481
Gross unrealized losses	(2)	0
Fair value	$ 474	$ 742