Investment Securities (Maturities of Securities Available for Sale and Held to Maturity) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Available for sale securities, fair value
Within 1 year	$ 7,568
After 1 year through 5 years	21,280
After 5 years through 10 years	7,889
After 10 years	11,325
Fair Value	48,062	$ 77,365
Available for sale securities, amortized cost
Within 1 year	7,563
After 1 year through 5 years	21,278
After 5 years through 10 years	7,881
After 10 years	11,301
Amortized Cost	48,023	77,627
Held to maturity securities, fair value
Within 1 year	553
After 1 year through 5 years	15,820
After 5 years through 10 years	35,654
After 10 years	62,305
Fair Value	114,332	74,444
Held to maturity securities, amortized cost
Within 1 year	553
After 1 year through 5 years	15,672
After 5 years through 10 years	35,558
After 10 years	62,593
Amortized Cost	114,376	75,203
U.S. agency mortgage-backed securities [Member]
Available for sale securities, fair value
Within 1 year	83
After 1 year through 5 years	2,390
After 5 years through 10 years	6,646
After 10 years	10,627
Fair Value	19,746	33,195
Available for sale securities, amortized cost
Amortized Cost	19,717	33,167
Held to maturity securities, fair value
Within 1 year	303
After 1 year through 5 years	11,401
After 5 years through 10 years	29,606
After 10 years	54,506
Fair Value	95,816	71,677
Held to maturity securities, amortized cost
Amortized Cost	96,045	72,439
Asset-backed securities [Member]
Available for sale securities, fair value
Within 1 year	0
After 1 year through 5 years	8,096
After 5 years through 10 years	1,243
After 10 years	657
Fair Value	9,996	20,335
Available for sale securities, amortized cost
Amortized Cost	9,960	20,520
Held to maturity securities, fair value
Within 1 year	0
After 1 year through 5 years	1,016
After 5 years through 10 years	5,364
After 10 years	5,956
Fair Value	12,336	941
Held to maturity securities, amortized cost
Amortized Cost	12,237	941
Corporate debt securities [Member]
Available for sale securities, fair value
Within 1 year	3,381
After 1 year through 5 years	3,087
After 5 years through 10 years	0
After 10 years	0
Fair Value	6,468	9,852
Available for sale securities, amortized cost
Amortized Cost	6,449	9,850
Held to maturity securities, fair value
Within 1 year	250
After 1 year through 5 years	3,153
After 5 years through 10 years	0
After 10 years	0
Fair Value	3,403	436
Held to maturity securities, amortized cost
Amortized Cost	3,377	436
U.S. Treasury securities [Member]
Available for sale securities, fair value
Within 1 year	2,069
After 1 year through 5 years	5,630
After 5 years through 10 years	0
After 10 years	0
Fair Value	7,699	8,623
Available for sale securities, amortized cost
Amortized Cost	7,741	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	876
After 1 year through 5 years	967
After 5 years through 10 years	0
After 10 years	0
Fair Value	1,843	2,071
Available for sale securities, amortized cost
Amortized Cost	1,840	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	1,913	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	98
After 10 years	1,201
Fair Value	1,299	68
Held to maturity securities, amortized cost
Amortized Cost	1,249	68
Commercial paper [Member]
Available for sale securities, fair value
Within 1 year	312
After 1 year through 5 years	0
After 5 years through 10 years	0
After 10 years	0
Fair Value	312	214
Available for sale securities, amortized cost
Amortized Cost	312	214
Held to maturity securities, fair value
Fair Value		99
Held to maturity securities, amortized cost
Amortized Cost		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	41
Fair Value	41	45
Available for sale securities, amortized cost
Amortized Cost	41	45
Held to maturity securities, fair value
Within 1 year	0
After 1 year through 5 years	0
After 5 years through 10 years	364
After 10 years	642
Fair Value	1,006	1,004
Held to maturity securities, amortized cost
Amortized Cost	995	$ 997
Foreign government agency securities [Member]
Available for sale securities, fair value
Within 1 year	0
After 1 year through 5 years	50
After 5 years through 10 years	0
After 10 years	0
Fair Value	50
Available for sale securities, amortized cost
Amortized Cost	50
Held to maturity securities, fair value
Within 1 year	0
After 1 year through 5 years	50
After 5 years through 10 years	0
After 10 years	0
Fair Value	50
Held to maturity securities, amortized cost
Amortized Cost	$ 50