Investment Securities (Maturities of Securities Available for Sale and Held to Maturity) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Available for sale securities, fair value
Within 1 year	$ 8,508
After 1 year through 5 years	20,960
After 5 years through 10 years	9,543
After 10 years	10,984
Fair Value	49,995	$ 77,365
Available for sale securities, amortized cost
Within 1 year	8,517
After 1 year through 5 years	20,999
After 5 years through 10 years	9,546
After 10 years	10,967
Amortized Cost	$ 50,029	77,627
Weighted-average yield
Within 1 year	1.53%
After 1 year through 5 years	1.63%
After 5 years through 10 years	1.72%
After 10 years	1.79%
Total	1.66%
Held to maturity securities, fair value
Within 1 year	$ 792
After 1 year through 5 years	17,500
After 5 years through 10 years	36,551
After 10 years	65,530
Fair Value	120,373	74,444
Held to maturity securities, amortized cost
Within 1 year	792
After 1 year through 5 years	17,486
After 5 years through 10 years	36,544
After 10 years	66,104
Amortized Cost	$ 120,926	75,203
Weighted-average yield
Within 1 year	1.97%
After 1 year through 5 years	2.45%
After 5 years through 10 years	2.35%
After 10 years	2.16%
Total	2.26%
U.S. agency mortgage-backed securities [Member]
Available for sale securities, fair value
Within 1 year	$ 61
After 1 year through 5 years	2,253
After 5 years through 10 years	8,282
After 10 years	10,333
Fair Value	20,929	33,195
Available for sale securities, amortized cost
Amortized Cost	20,915	33,167
Held to maturity securities, fair value
Within 1 year	441
After 1 year through 5 years	12,680
After 5 years through 10 years	29,511
After 10 years	57,821
Fair Value	100,453	71,677
Held to maturity securities, amortized cost
Amortized Cost	101,197	72,439
Asset-backed securities [Member]
Available for sale securities, fair value
Within 1 year	251
After 1 year through 5 years	6,924
After 5 years through 10 years	1,261
After 10 years	611
Fair Value	9,047	20,335
Available for sale securities, amortized cost
Amortized Cost	9,019	20,520
Held to maturity securities, fair value
Within 1 year	0
After 1 year through 5 years	1,003
After 5 years through 10 years	6,245
After 10 years	5,814
Fair Value	13,062	941
Held to maturity securities, amortized cost
Amortized Cost	12,937	941
Corporate debt securities [Member]
Available for sale securities, fair value
Within 1 year	3,135
After 1 year through 5 years	3,034
After 5 years through 10 years	0
After 10 years	0
Fair Value	6,169	9,852
Available for sale securities, amortized cost
Amortized Cost	6,154	9,850
Held to maturity securities, fair value
Within 1 year	351
After 1 year through 5 years	3,206
After 5 years through 10 years	454
After 10 years	75
Fair Value	4,086	436
Held to maturity securities, amortized cost
Amortized Cost	4,078	436
U.S. Treasury securities [Member]
Available for sale securities, fair value
Within 1 year	2,515
After 1 year through 5 years	6,985
After 5 years through 10 years	0
After 10 years	0
Fair Value	9,500	8,623
Available for sale securities, amortized cost
Amortized Cost	9,583	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	220
After 10 years	0
Fair Value	220	219
Held to maturity securities, amortized cost
Amortized Cost	223	223
Certificates of deposit [Member]
Available for sale securities, fair value
Within 1 year	575
After 1 year through 5 years	1,466
After 5 years through 10 years	0
After 10 years	0
Fair Value	2,041	2,071
Available for sale securities, amortized cost
Amortized Cost	2,040	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	1,658
After 1 year through 5 years	248
After 5 years through 10 years	0
After 10 years	0
Fair Value	1,906	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	121
After 10 years	1,183
Fair Value	1,304	68
Held to maturity securities, amortized cost
Amortized Cost	1,247	68
Commercial paper [Member]
Available for sale securities, fair value
Within 1 year	313
After 1 year through 5 years	0
After 5 years through 10 years	0
After 10 years	0
Fair Value	313	214
Available for sale securities, amortized cost
Amortized Cost	313	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	40
Fair Value	40	45
Available for sale securities, amortized cost
Amortized Cost	40	45
Held to maturity securities, fair value
Within 1 year	0
After 1 year through 5 years	362
After 5 years through 10 years	0
After 10 years	637
Fair Value	999	1,004
Held to maturity securities, amortized cost
Amortized Cost	994	$ 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	51
Held to maturity securities, fair value
Within 1 year	0
After 1 year through 5 years	49
After 5 years through 10 years	0
After 10 years	0
Fair Value	49
Held to maturity securities, amortized cost
Amortized Cost	$ 50