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	Sep. 30, 2018 USD ($) security	Dec. 31, 2017 USD ($) security
Available for sale securities:
Less than 12 months Fair Value	$ 23,326	$ 12,909
Less than 12 months Unrealized Losses	154	38
12 months or longer Fair Value	8,356	9,774
12 months or longer Unrealized Losses	144	101
Total Fair Value	31,682	22,683
Total Unrealized Losses	298	139
Held to maturity securities:
Less than 12 months Fair Value	65,742	45,180
Less than 12 months Unrealized Losses	1,331	326
12 months or longer Fair Value	40,948	24,825
12 months or longer Unrealized Losses	2,264	724
Total Fair Value	106,690	70,005
Total Unrealized Losses	3,595	1,050
Total securities with unrealized losses
Less than 12 months Fair Value	89,068	58,089
Less than 12 months Unrealized Losses	1,485	364
12 months or longer Fair Value	49,304	34,599
12 months or longer Unrealized Losses	2,408	825
Total Fair Value	138,372	92,688
Total Unrealized Losses	$ 3,893	$ 1,189
Number of available for sale securities in unrealized loss positions | security	413	251
Number of held to maturity securities in unrealized loss positions | security	1,800	938
U.S. agency mortgage-backed securities [Member]
Available for sale securities:
Less than 12 months Fair Value	$ 10,119	$ 5,696
Less than 12 months Unrealized Losses	68	21
12 months or longer Fair Value	1,988	2,548
12 months or longer Unrealized Losses	34	18
Total Fair Value	12,107	8,244
Total Unrealized Losses	102	39
Held to maturity securities:
Less than 12 months Fair Value	59,643	42,102
Less than 12 months Unrealized Losses	1,247	310
12 months or longer Fair Value	40,279	24,753
12 months or longer Unrealized Losses	2,236	724
Total Fair Value	99,922	66,855
Total Unrealized Losses	3,483	1,034
U.S. Treasury securities [Member]
Available for sale securities:
Less than 12 months Fair Value	7,770	4,625
Less than 12 months Unrealized Losses	72	11
12 months or longer Fair Value	4,374	4,875
12 months or longer Unrealized Losses	97	72
Total Fair Value	12,144	9,500
Total Unrealized Losses	169	83
Held to maturity securities:
Less than 12 months Fair Value	0	220
Less than 12 months Unrealized Losses	0	3
12 months or longer Fair Value	212	0
12 months or longer Unrealized Losses	11	0
Total Fair Value	212	220
Total Unrealized Losses	11	3
Asset-backed securities [Member]
Available for sale securities:
Less than 12 months Fair Value	1,841	904
Less than 12 months Unrealized Losses	4	3
12 months or longer Fair Value	601	424
12 months or longer Unrealized Losses	6	3
Total Fair Value	2,442	1,328
Total Unrealized Losses	10	6
Held to maturity securities:
Less than 12 months Fair Value	2,115	1,124
Less than 12 months Unrealized Losses	12	2
12 months or longer Fair Value	39	72
12 months or longer Unrealized Losses	0	0
Total Fair Value	2,154	1,196
Total Unrealized Losses	12	2
Corporate debt securities [Member]
Available for sale securities:
Less than 12 months Fair Value	3,030	736
Less than 12 months Unrealized Losses	8	1
12 months or longer Fair Value	229	120
12 months or longer Unrealized Losses	1	0
Total Fair Value	3,259	856
Total Unrealized Losses	9	1
Held to maturity securities:
Less than 12 months Fair Value	2,862	1,078
Less than 12 months Unrealized Losses	52	5
12 months or longer Fair Value	77	0
12 months or longer Unrealized Losses	3	0
Total Fair Value	2,939	1,078
Total Unrealized Losses	55	5
Certificates of deposit [Member]
Available for sale securities:
Less than 12 months Fair Value		799
Less than 12 months Unrealized Losses		1
12 months or longer Fair Value		0
12 months or longer Unrealized Losses		0
Total Fair Value		799
Total Unrealized Losses		1
U.S. agency notes [Member]
Available for sale securities:
Less than 12 months Fair Value	566	99
Less than 12 months Unrealized Losses	2	0
12 months or longer Fair Value	1,116	1,807
12 months or longer Unrealized Losses	4	8
Total Fair Value	1,682	1,906
Total Unrealized Losses	6	8
Foreign government agency securities [Member]
Available for sale securities:
Less than 12 months Fair Value	0	50
Less than 12 months Unrealized Losses	0	1
12 months or longer Fair Value	48	0
12 months or longer Unrealized Losses	2	0
Total Fair Value	48	50
Total Unrealized Losses	2	1
Held to maturity securities:
Less than 12 months Fair Value	0	49
Less than 12 months Unrealized Losses	0	1
12 months or longer Fair Value	48	0
12 months or longer Unrealized Losses	2	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	471
Less than 12 months Unrealized Losses	5
12 months or longer Fair Value	14
12 months or longer Unrealized Losses	2
Total Fair Value	485
Total Unrealized Losses	7
Non-agency commercial mortgage-backed securities [Member]
Held to maturity securities:
Less than 12 months Fair Value	651	607
Less than 12 months Unrealized Losses	15	5
12 months or longer Fair Value	279	0
12 months or longer Unrealized Losses	10	0
Total Fair Value	930	607
Total Unrealized Losses	$ 25	$ 5