Investment Securities (Available For Sale and Held to Maturity Securities with Unrealized Losses, Aggregated by Category and Period of Continuous Unrealized Loss) (Details) $ in Millions	Jun. 30, 2018 USD ($) security	Dec. 31, 2017 USD ($) security
Available for sale securities:
Less than 12 months Fair Value	$ 18,951	$ 12,909
Less than 12 months Unrealized Losses	147	38
12 months or longer Fair Value	7,770	9,774
12 months or longer Unrealized Losses	112	101
Total Fair Value	26,721	22,683
Total Unrealized Losses	259	139
Held to maturity securities:
Less than 12 months Fair Value	74,510	45,180
Less than 12 months Unrealized Losses	1,641	326
12 months or longer Fair Value	25,009	24,825
12 months or longer Unrealized Losses	1,368	724
Total Fair Value	99,519	70,005
Total Unrealized Losses	3,009	1,050
Total securities with unrealized losses
Less than 12 months Fair Value	93,461	58,089
Less than 12 months Unrealized Losses	1,788	364
12 months or longer Fair Value	32,779	34,599
12 months or longer Unrealized Losses	1,480	825
Total Fair Value	126,240	92,688
Total Unrealized Losses	$ 3,268	$ 1,189
Number of available for sale securities in unrealized loss positions | security	332	251
Number of held to maturity securities in unrealized loss positions | security	1,543	938
U.S. agency mortgage-backed securities [Member]
Available for sale securities:
Less than 12 months Fair Value	$ 7,861	$ 5,696
Less than 12 months Unrealized Losses	68	21
12 months or longer Fair Value	1,732	2,548
12 months or longer Unrealized Losses	12	18
Total Fair Value	9,593	8,244
Total Unrealized Losses	80	39
Held to maturity securities:
Less than 12 months Fair Value	68,494	42,102
Less than 12 months Unrealized Losses	1,539	310
12 months or longer Fair Value	24,984	24,753
12 months or longer Unrealized Losses	1,368	724
Total Fair Value	93,478	66,855
Total Unrealized Losses	2,907	1,034
U.S. Treasury securities [Member]
Available for sale securities:
Less than 12 months Fair Value	5,639	4,625
Less than 12 months Unrealized Losses	62	11
12 months or longer Fair Value	4,556	4,875
12 months or longer Unrealized Losses	91	72
Total Fair Value	10,195	9,500
Total Unrealized Losses	153	83
Held to maturity securities:
Less than 12 months Fair Value	214	220
Less than 12 months Unrealized Losses	9	3
12 months or longer Fair Value	0	0
12 months or longer Unrealized Losses	0	0
Total Fair Value	214	220
Total Unrealized Losses	9	3
Asset-backed securities [Member]
Available for sale securities:
Less than 12 months Fair Value	2,495	904
Less than 12 months Unrealized Losses	7	3
12 months or longer Fair Value	348	424
12 months or longer Unrealized Losses	3	3
Total Fair Value	2,843	1,328
Total Unrealized Losses	10	6
Held to maturity securities:
Less than 12 months Fair Value	2,097	1,124
Less than 12 months Unrealized Losses	10	2
12 months or longer Fair Value	25	72
12 months or longer Unrealized Losses	0	0
Total Fair Value	2,122	1,196
Total Unrealized Losses	10	2
Corporate debt securities [Member]
Available for sale securities:
Less than 12 months Fair Value	2,163	736
Less than 12 months Unrealized Losses	7	1
12 months or longer Fair Value	20	120
12 months or longer Unrealized Losses	0	0
Total Fair Value	2,183	856
Total Unrealized Losses	7	1
Held to maturity securities:
Less than 12 months Fair Value	2,637	1,078
Less than 12 months Unrealized Losses	55	5
12 months or longer Fair Value	0	0
12 months or longer Unrealized Losses	0	0
Total Fair Value	2,637	1,078
Total Unrealized Losses	55	5
Certificates of deposit [Member]
Available for sale securities:
Less than 12 months Fair Value	549	799
Less than 12 months Unrealized Losses	1	1
12 months or longer Fair Value	0	0
12 months or longer Unrealized Losses	0	0
Total Fair Value	549	799
Total Unrealized Losses	1	1
U.S. agency notes [Member]
Available for sale securities:
Less than 12 months Fair Value	195	99
Less than 12 months Unrealized Losses	0	0
12 months or longer Fair Value	1,114	1,807
12 months or longer Unrealized Losses	6	8
Total Fair Value	1,309	1,906
Total Unrealized Losses	6	8
Foreign government agency securities [Member]
Available for sale securities:
Less than 12 months Fair Value	49	50
Less than 12 months Unrealized Losses	2	1
12 months or longer Fair Value	0	0
12 months or longer Unrealized Losses	0	0
Total Fair Value	49	50
Total Unrealized Losses	2	1
Held to maturity securities:
Less than 12 months Fair Value	48	49
Less than 12 months Unrealized Losses	2	1
12 months or longer Fair Value	0	0
12 months or longer Unrealized Losses	0	0
Total Fair Value	48	49
Total Unrealized Losses	2	1
U.S. state and municipal securities [Member]
Held to maturity securities:
Less than 12 months Fair Value	118
Less than 12 months Unrealized Losses	3
12 months or longer Fair Value	0
12 months or longer Unrealized Losses	0
Total Fair Value	118
Total Unrealized Losses	3
Non-agency commercial mortgage-backed securities [Member]
Held to maturity securities:
Less than 12 months Fair Value	902	607
Less than 12 months Unrealized Losses	23	5
12 months or longer Fair Value	0	0
12 months or longer Unrealized Losses	0	0
Total Fair Value	902	607
Total Unrealized Losses	$ 23	$ 5