Investment Securities (Maturities of Securities Available for Sale and Held to Maturity) (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Available for sale securities, fair value
Within 1 year	$ 7,828
After 1 year through 5 years	26,907
After 5 years through 10 years	10,504
After 10 years	10,283
Fair Value	55,522	$ 49,995
Available for sale securities, amortized cost
Within 1 year	7,836
After 1 year through 5 years	27,038
After 5 years through 10 years	10,533
After 10 years	10,290
Amortized Cost	55,697	50,029
Held to maturity securities, fair value
Within 1 year	715
After 1 year through 5 years	18,958
After 5 years through 10 years	41,679
After 10 years	72,640
Fair Value	133,992	120,373
Held to maturity securities, amortized cost
Within 1 year	716
After 1 year through 5 years	19,252
After 5 years through 10 years	42,448
After 10 years	74,376
Amortized Cost	136,792	120,926
U.S. agency mortgage-backed securities [Member]
Available for sale securities, fair value
Within 1 year	93
After 1 year through 5 years	3,664
After 5 years through 10 years	9,498
After 10 years	9,692
Fair Value	22,947	20,929
Available for sale securities, amortized cost
Amortized Cost	22,977	20,915
Held to maturity securities, fair value
Within 1 year	322
After 1 year through 5 years	13,730
After 5 years through 10 years	31,739
After 10 years	64,463
Fair Value	110,254	100,453
Held to maturity securities, amortized cost
Amortized Cost	113,106	101,197
U.S. Treasury securities [Member]
Available for sale securities, fair value
Within 1 year	3,194
After 1 year through 5 years	7,665
After 5 years through 10 years	0
After 10 years	0
Fair Value	10,859	9,500
Available for sale securities, amortized cost
Amortized Cost	11,012	9,583
Held to maturity securities, fair value
Within 1 year	0
After 1 year through 5 years	0
After 5 years through 10 years	214
After 10 years	0
Fair Value	214	220
Held to maturity securities, amortized cost
Amortized Cost	223	223
Asset-backed securities [Member]
Available for sale securities, fair value
Within 1 year	250
After 1 year through 5 years	8,909
After 5 years through 10 years	1,006
After 10 years	555
Fair Value	10,720	9,047
Available for sale securities, amortized cost
Amortized Cost	10,710	9,019
Held to maturity securities, fair value
Within 1 year	0
After 1 year through 5 years	1,083
After 5 years through 10 years	8,978
After 10 years	6,410
Fair Value	16,471	13,062
Held to maturity securities, amortized cost
Amortized Cost	16,356	12,937
Corporate debt securities [Member]
Available for sale securities, fair value
Within 1 year	2,253
After 1 year through 5 years	3,938
After 5 years through 10 years	0
After 10 years	0
Fair Value	6,191	6,169
Available for sale securities, amortized cost
Amortized Cost	6,187	6,154
Held to maturity securities, fair value
Within 1 year	393
After 1 year through 5 years	3,543
After 5 years through 10 years	568
After 10 years	0
Fair Value	4,504	4,086
Held to maturity securities, amortized cost
Amortized Cost	4,550	4,078
Certificates of deposit [Member]
Available for sale securities, fair value
Within 1 year	672
After 1 year through 5 years	2,020
After 5 years through 10 years	0
After 10 years	0
Fair Value	2,692	2,041
Available for sale securities, amortized cost
Amortized Cost	2,690	2,040
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	200
Held to maturity securities, amortized cost
Amortized Cost	200	200
U.S. agency notes [Member]
Available for sale securities, fair value
Within 1 year	861
After 1 year through 5 years	663
After 5 years through 10 years	0
After 10 years	0
Fair Value	1,524	1,906
Available for sale securities, amortized cost
Amortized Cost	1,530	1,914
U.S. state and municipal securities [Member]
Held to maturity securities, fair value
Within 1 year	0
After 1 year through 5 years	0
After 5 years through 10 years	180
After 10 years	1,077
Fair Value	1,257	1,304
Held to maturity securities, amortized cost
Amortized Cost	1,242	1,247
Commercial paper [Member]
Available for sale securities, fair value
Within 1 year	505
After 1 year through 5 years	0
After 5 years through 10 years	0
After 10 years	0
Fair Value	505	313
Available for sale securities, amortized cost
Amortized Cost	505	313
Foreign government agency securities [Member]
Available for sale securities, fair value
Within 1 year	0
After 1 year through 5 years	48
After 5 years through 10 years	0
After 10 years	0
Fair Value	48	50
Available for sale securities, amortized cost
Amortized Cost	50	51
Held to maturity securities, fair value
Within 1 year	0
After 1 year through 5 years	48
After 5 years through 10 years	0
After 10 years	0
Fair Value	48	49
Held to maturity securities, amortized cost
Amortized Cost	50	50
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	36
Fair Value	36	40
Available for sale securities, amortized cost
Amortized Cost	36	40
Held to maturity securities, fair value
Within 1 year	0
After 1 year through 5 years	354
After 5 years through 10 years	0
After 10 years	690
Fair Value	1,044	999
Held to maturity securities, amortized cost
Amortized Cost	$ 1,065	$ 994