Investment Securities (Maturities of Securities Available for Sale and Held to Maturity) (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Available for sale securities, fair value
Within 1 year	$ 5,749
After 1 year through 5 years	11,065
After 5 years through 10 years	12,026
After 10 years	27,643
Fair Value	56,483	$ 66,578
Available for sale securities, amortized cost
Within 1 year	5,743
After 1 year through 5 years	10,994
After 5 years through 10 years	11,996
After 10 years	27,416
Amortized Cost	56,149	66,735
Held to maturity securities, fair value
Within 1 year	1,641
After 1 year through 5 years	22,294
After 5 years through 10 years	44,528
After 10 years	73,700
Fair Value	142,163	142,038
Held to maturity securities, amortized cost
Within 1 year	1,637
After 1 year through 5 years	21,989
After 5 years through 10 years	43,368
After 10 years	73,200
Amortized Cost	140,194	144,009
U.S. agency mortgage-backed securities [Member]
Available for sale securities, fair value
Within 1 year	51
After 1 year through 5 years	1,909
After 5 years through 10 years	11,061
After 10 years	27,410
Fair Value	40,431	25,556
Available for sale securities, amortized cost
Amortized Cost	40,179	25,594
Held to maturity securities, fair value
Within 1 year	663
After 1 year through 5 years	15,472
After 5 years through 10 years	35,048
After 10 years	64,829
Fair Value	116,012	116,093
Held to maturity securities, amortized cost
Amortized Cost	114,276	118,064
U.S. Treasury securities [Member]
Available for sale securities, fair value
Within 1 year	3,541
After 1 year through 5 years	1,486
After 5 years through 10 years	0
After 10 years	0
Fair Value	5,027	18,302
Available for sale securities, amortized cost
Amortized Cost	5,034	18,410
Held to maturity securities, fair value
Within 1 year	0
After 1 year through 5 years	0
After 5 years through 10 years	230
After 10 years	0
Fair Value	230	217
Held to maturity securities, amortized cost
Amortized Cost	224	223
Asset-backed securities [Member]
Available for sale securities, fair value
Within 1 year	35
After 1 year through 5 years	3,752
After 5 years through 10 years	794
After 10 years	220
Fair Value	4,801	10,085
Available for sale securities, amortized cost
Amortized Cost	4,773	10,086
Held to maturity securities, fair value
Within 1 year	0
After 1 year through 5 years	3,227
After 5 years through 10 years	7,940
After 10 years	7,192
Fair Value	18,359	18,546
Held to maturity securities, amortized cost
Amortized Cost	18,323	18,502
Corporate debt securities [Member]
Available for sale securities, fair value
Within 1 year	1,100
After 1 year through 5 years	3,415
After 5 years through 10 years	171
After 10 years	0
Fair Value	4,686	7,467
Available for sale securities, amortized cost
Amortized Cost	4,630	7,477
Held to maturity securities, fair value
Within 1 year	778
After 1 year through 5 years	3,079
After 5 years through 10 years	858
After 10 years	0
Fair Value	4,715	4,432
Held to maturity securities, amortized cost
Amortized Cost	4,659	4,477
Certificates of deposit [Member]
Available for sale securities, fair value
Within 1 year	501
After 1 year through 5 years	503
After 5 years through 10 years	0
After 10 years	0
Fair Value	1,004	3,685
Available for sale securities, amortized cost
Amortized Cost	1,000	3,682
Held to maturity securities, fair value
Within 1 year	200
After 1 year through 5 years	0
After 5 years through 10 years	0
After 10 years	0
Fair Value	200	201
Held to maturity securities, amortized cost
Amortized Cost	200	200
U.S. state and municipal securities [Member]
Held to maturity securities, fair value
Within 1 year	0
After 1 year through 5 years	99
After 5 years through 10 years	452
After 10 years	862
Fair Value	1,413	1,348
Held to maturity securities, amortized cost
Amortized Cost	1,306	1,327
Commercial paper [Member]
Available for sale securities, fair value
Within 1 year	521
After 1 year through 5 years	0
After 5 years through 10 years	0
After 10 years	0
Fair Value	521	522
Available for sale securities, amortized cost
Amortized Cost	520	522
Foreign government agency securities [Member]
Available for sale securities, fair value
Fair Value		49
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	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	13
Fair Value	13	14
Available for sale securities, amortized cost
Amortized Cost	13	14
Held to maturity securities, fair value
Within 1 year	0
After 1 year through 5 years	367
After 5 years through 10 years	0
After 10 years	797
Fair Value	1,164	1,142
Held to maturity securities, amortized cost
Amortized Cost	1,136	1,156
Other [Member]
Held to maturity securities, fair value
Within 1 year	0
After 1 year through 5 years	0
After 5 years through 10 years	0
After 10 years	20
Fair Value	20	10
Held to maturity securities, amortized cost
Amortized Cost	$ 20	$ 10