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	Dec. 31, 2017 USD ($) security	Dec. 31, 2016 USD ($) security
Available for sale securities:
Less than 12 months Fair Value	$ 12,909	$ 29,156
Less than 12 months Unrealized Losses	38	212
12 months or longer Fair Value	9,774	12,921
12 months or longer Unrealized Losses	101	226
Total Fair Value	22,683	42,077
Total Unrealized Losses	139	438
Held to maturity securities:
Less than 12 months Fair Value	45,180	52,185
Less than 12 months Unrealized Losses	326	1,095
12 months or longer Fair Value	24,825	0
12 months or longer Unrealized Losses	724	0
Total Fair Value	70,005	52,185
Total Unrealized Losses	1,050	1,095
Total securities with unrealized losses
Less than 12 months Fair Value	58,089	81,341
Less than 12 months Unrealized Losses	364	1,307
12 months or longer Fair Value	34,599	12,921
12 months or longer Unrealized Losses	825	226
Total Fair Value	92,688	94,262
Total Unrealized Losses	$ 1,189	$ 1,533
Number of available for sale securities in unrealized loss positions | security	251	627
Number of held to maturity securities in unrealized loss positions | security	938	612
U.S. agency mortgage-backed securities [Member]
Available for sale securities:
Less than 12 months Fair Value	$ 5,696	$ 14,816
Less than 12 months Unrealized Losses	21	69
12 months or longer Fair Value	2,548	2,931
12 months or longer Unrealized Losses	18	23
Total Fair Value	8,244	17,747
Total Unrealized Losses	39	92
Held to maturity securities:
Less than 12 months Fair Value	42,102	51,361
Less than 12 months Unrealized Losses	310	1,086
12 months or longer Fair Value	24,753	0
12 months or longer Unrealized Losses	724	0
Total Fair Value	66,855	51,361
Total Unrealized Losses	1,034	1,086
Non-agency commercial mortgage-backed securities [Member]
Held to maturity securities:
Less than 12 months Fair Value	607	591
Less than 12 months Unrealized Losses	5	4
12 months or longer Fair Value	0	0
12 months or longer Unrealized Losses	0	0
Total Fair Value	607	591
Total Unrealized Losses	5	4
Asset-backed securities [Member]
Available for sale securities:
Less than 12 months Fair Value	904	1,670
Less than 12 months Unrealized Losses	3	13
12 months or longer Fair Value	424	9,237
12 months or longer Unrealized Losses	3	201
Total Fair Value	1,328	10,907
Total Unrealized Losses	6	214
Held to maturity securities:
Less than 12 months Fair Value	1,124
Less than 12 months Unrealized Losses	2
12 months or longer Fair Value	72
12 months or longer Unrealized Losses	0
Total Fair Value	1,196
Total Unrealized Losses	2
Corporate debt securities [Member]
Available for sale securities:
Less than 12 months Fair Value	736	2,407
Less than 12 months Unrealized Losses	1	17
12 months or longer Fair Value	120	653
12 months or longer Unrealized Losses	0	1
Total Fair Value	856	3,060
Total Unrealized Losses	1	18
Held to maturity securities:
Less than 12 months Fair Value	1,078
Less than 12 months Unrealized Losses	5
12 months or longer Fair Value	0
12 months or longer Unrealized Losses	0
Total Fair Value	1,078
Total Unrealized Losses	5
U.S. Treasury securities [Member]
Available for sale securities:
Less than 12 months Fair Value	4,625	6,926
Less than 12 months Unrealized Losses	11	59
12 months or longer Fair Value	4,875	0
12 months or longer Unrealized Losses	72	0
Total Fair Value	9,500	6,926
Total Unrealized Losses	83	59
Held to maturity securities:
Less than 12 months Fair Value	220	219
Less than 12 months Unrealized Losses	3	4
12 months or longer Fair Value	0	0
12 months or longer Unrealized Losses	0	0
Total Fair Value	220	219
Total Unrealized Losses	3	4
Certificates of deposit [Member]
Available for sale securities:
Less than 12 months Fair Value	799	474
Less than 12 months Unrealized Losses	1	0
12 months or longer Fair Value	0	100
12 months or longer Unrealized Losses	0	1
Total Fair Value	799	574
Total Unrealized Losses	1	1
U.S. agency notes [Member]
Available for sale securities:
Less than 12 months Fair Value	99	1,907
Less than 12 months Unrealized Losses	0	8
12 months or longer Fair Value	1,807	0
12 months or longer Unrealized Losses	8	0
Total Fair Value	1,906	1,907
Total Unrealized Losses	8	8
Foreign government agency securities [Member]
Available for sale securities:
Less than 12 months Fair Value	50
Less than 12 months Unrealized Losses	1
12 months or longer Fair Value	0
12 months or longer Unrealized Losses	0
Total Fair Value	50
Total Unrealized Losses	1
Held to maturity securities:
Less than 12 months Fair Value	49
Less than 12 months Unrealized Losses	1
12 months or longer Fair Value	0
12 months or longer Unrealized Losses	0
Total Fair Value	49
Total Unrealized Losses	$ 1
U.S. state and municipal securities [Member]
Available for sale securities:
Less than 12 months Fair Value		956
Less than 12 months Unrealized Losses		46
12 months or longer Fair Value		0
12 months or longer Unrealized Losses		0
Total Fair Value		956
Total Unrealized Losses		46
Held to maturity securities:
Less than 12 months Fair Value		14
Less than 12 months Unrealized Losses		1
12 months or longer Fair Value		0
12 months or longer Unrealized Losses		0
Total Fair Value		14
Total Unrealized Losses		$ 1