Investment Securities (Maturities of Securities Available for Sale and Held to Maturity) (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Available for sale securities, fair value
Within 1 year	$ 6,613
After 1 year through 5 years	21,486
After 5 years through 10 years	7,432
After 10 years	10,103
Fair Value	45,634	$ 77,365
Available for sale securities, amortized cost
Within 1 year	6,612
After 1 year through 5 years	21,476
After 5 years through 10 years	7,427
After 10 years	10,080
Amortized Cost	45,595	77,627
Held to maturity securities, fair value
Within 1 year	100
After 1 year through 5 years	14,118
After 5 years through 10 years	36,292
After 10 years	56,936
Fair Value	107,446	74,444
Held to maturity securities, amortized cost
Within 1 year	100
After 1 year through 5 years	13,939
After 5 years through 10 years	36,222
After 10 years	57,349
Amortized Cost	107,610	75,203
U.S. agency mortgage-backed securities [Member]
Available for sale securities, fair value
Within 1 year	108
After 1 year through 5 years	1,884
After 5 years through 10 years	6,306
After 10 years	9,335
Fair Value	17,633	33,195
Available for sale securities, amortized cost
Amortized Cost	17,604	33,167
Held to maturity securities, fair value
Within 1 year	0
After 1 year through 5 years	9,695
After 5 years through 10 years	30,224
After 10 years	49,025
Fair Value	88,944	71,677
Held to maturity securities, amortized cost
Amortized Cost	89,250	72,439
Asset-backed securities [Member]
Available for sale securities, fair value
Within 1 year	23
After 1 year through 5 years	8,019
After 5 years through 10 years	1,126
After 10 years	726
Fair Value	9,894	20,335
Available for sale securities, amortized cost
Amortized Cost	9,858	20,520
Held to maturity securities, fair value
Within 1 year	0
After 1 year through 5 years	1,019
After 5 years through 10 years	5,395
After 10 years	6,147
Fair Value	12,561	941
Held to maturity securities, amortized cost
Amortized Cost	12,493	941
Corporate debt securities [Member]
Available for sale securities, fair value
Within 1 year	2,851
After 1 year through 5 years	3,774
After 5 years through 10 years	0
After 10 years	0
Fair Value	6,625	9,852
Available for sale securities, amortized cost
Amortized Cost	6,603	9,850
Held to maturity securities, fair value
Within 1 year	0
After 1 year through 5 years	3,204
After 5 years through 10 years	0
After 10 years	0
Fair Value	3,204	436
Held to maturity securities, amortized cost
Amortized Cost	3,181	436
U.S. Treasury securities [Member]
Available for sale securities, fair value
Within 1 year	1,619
After 1 year through 5 years	6,078
After 5 years through 10 years	0
After 10 years	0
Fair Value	7,697	8,623
Available for sale securities, amortized cost
Amortized Cost	7,740	8,679
Held to maturity securities, fair value
Within 1 year	0
After 1 year through 5 years	0
After 5 years through 10 years	222
After 10 years	0
Fair Value	222	219
Held to maturity securities, amortized cost
Amortized Cost	223	223
Certificates of deposit [Member]
Available for sale securities, fair value
Within 1 year	951
After 1 year through 5 years	671
After 5 years through 10 years	0
After 10 years	0
Fair Value	1,622	2,071
Available for sale securities, amortized cost
Amortized Cost	1,620	2,070
Held to maturity securities, fair value
Within 1 year	0
After 1 year through 5 years	200
After 5 years through 10 years	0
After 10 years	0
Fair Value	200
Held to maturity securities, amortized cost
Amortized Cost	200
U.S. agency notes [Member]
Available for sale securities, fair value
Within 1 year	847
After 1 year through 5 years	1,060
After 5 years through 10 years	0
After 10 years	0
Fair Value	1,907	1,907
Available for sale securities, amortized cost
Amortized Cost	1,914	1,915
U.S. state and municipal securities [Member]
Available for sale securities, fair value
Fair Value		1,123
Available for sale securities, amortized cost
Amortized Cost		1,167
Held to maturity securities, fair value
Within 1 year	0
After 1 year through 5 years	0
After 5 years through 10 years	88
After 10 years	1,123
Fair Value	1,211	68
Held to maturity securities, amortized cost
Amortized Cost	1,168	68
Commercial paper [Member]
Available for sale securities, fair value
Within 1 year	214
After 1 year through 5 years	0
After 5 years through 10 years	0
After 10 years	0
Fair Value	214	214
Available for sale securities, amortized cost
Amortized Cost	214	214
Held to maturity securities, fair value
Within 1 year	100
After 1 year through 5 years	0
After 5 years through 10 years	0
After 10 years	0
Fair Value	100	99
Held to maturity securities, amortized cost
Amortized Cost	100	99
Non-agency commercial mortgage-backed securities [Member]
Available for sale securities, fair value
Within 1 year	0
After 1 year through 5 years	0
After 5 years through 10 years	0
After 10 years	42
Fair Value	42	45
Available for sale securities, amortized cost
Amortized Cost	42	45
Held to maturity securities, fair value
Within 1 year	0
After 1 year through 5 years	0
After 5 years through 10 years	363
After 10 years	641
Fair Value	1,004	1,004
Held to maturity securities, amortized cost
Amortized Cost	$ 995	$ 997