Investment Securities - Schedule of Gross Pre-Tax Unrealized Losses on Investment Securities (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	$ 18,094	$ 16,928
Available for sale, gross unrealized losses less than 12 months	131	308
Available for sale, fair value 12 months or longer	6,391	7,800
Available for sale, gross unrealized losses 12 months or longer	137	177
Available for sale, fair value total	24,485	24,728
Available for sale, gross unrealized losses total	268	485
Held to maturity, fair value less than 12 months	21,664	17,284
Held-to-maturity, gross, less than 12 months	150	293
Held to maturity, fair value 12 months or longer	9,373	1,899
Held to maturity, gross unrealized losses 12 months or longer	239	42
Held to maturity, fair value total	31,037	19,183
Held to maturity, gross unrealized losses total	389	335
US Treasury and federal agencies, direct obligations
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	0	651
Available for sale, gross unrealized losses less than 12 months	0	8
Available for sale, fair value 12 months or longer	67	180
Available for sale, gross unrealized losses 12 months or longer	1	1
Available for sale, fair value total	67	831
Available for sale, gross unrealized losses total	1	9
Held to maturity, fair value less than 12 months	14,439	8,891
Held-to-maturity, gross, less than 12 months	109	57
Held to maturity, fair value 12 months or longer	2,447	86
Held to maturity, gross unrealized losses 12 months or longer	34	1
Held to maturity, fair value total	16,886	8,977
Held to maturity, gross unrealized losses total	143	58
US Treasury and federal agencies, mortgage-backed securities
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	5,161	7,072
Available for sale, gross unrealized losses less than 12 months	31	131
Available for sale, fair value 12 months or longer	3,341	1,114
Available for sale, gross unrealized losses 12 months or longer	98	28
Available for sale, fair value total	8,502	8,186
Available for sale, gross unrealized losses total	129	159
Held to maturity, fair value less than 12 months	6,785	6,838
Held-to-maturity, gross, less than 12 months	38	221
Held to maturity, fair value 12 months or longer	5,988	0
Held to maturity, gross unrealized losses 12 months or longer	187	0
Held to maturity, fair value total	12,773	6,838
Held to maturity, gross unrealized losses total	225	221
Total U.S. Treasury and federal agencies
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	5,161	7,723
Available for sale, gross unrealized losses less than 12 months	31	139
Available for sale, fair value 12 months or longer	3,408	1,294
Available for sale, gross unrealized losses 12 months or longer	99	29
Available for sale, fair value total	8,569	9,017
Available for sale, gross unrealized losses total	130	168
Held to maturity, fair value less than 12 months	21,224	15,729
Held-to-maturity, gross, less than 12 months	147	278
Held to maturity, fair value 12 months or longer	8,435	86
Held to maturity, gross unrealized losses 12 months or longer	221	1
Held to maturity, fair value total	29,659	15,815
Held to maturity, gross unrealized losses total	368	279
Asset-backed securities, student loans
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	0	54
Available for sale, gross unrealized losses less than 12 months	0	0
Available for sale, fair value 12 months or longer	769	3,745
Available for sale, gross unrealized losses 12 months or longer	4	75
Available for sale, fair value total	769	3,799
Available for sale, gross unrealized losses total	4	75
Held to maturity, fair value less than 12 months	440	705
Held-to-maturity, gross, less than 12 months	3	9
Held to maturity, fair value 12 months or longer	423	1,235
Held to maturity, gross unrealized losses 12 months or longer	6	21
Held to maturity, fair value total	863	1,940
Held to maturity, gross unrealized losses total	9	30
Asset-backed securities, credit cards
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	1,289	795
Available for sale, gross unrealized losses less than 12 months	25	1
Available for sale, fair value 12 months or longer	0	494
Available for sale, gross unrealized losses 12 months or longer	0	25
Available for sale, fair value total	1,289	1,289
Available for sale, gross unrealized losses total	25	26
Held to maturity, fair value less than 12 months		33
Held-to-maturity, gross, less than 12 months		0
Held to maturity, fair value 12 months or longer		0
Held to maturity, gross unrealized losses 12 months or longer		0
Held to maturity, fair value total		33
Held to maturity, gross unrealized losses total		0
Asset-backed securities, sub-prime
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months		1
Available for sale, gross unrealized losses less than 12 months		0
Available for sale, fair value 12 months or longer		252
Available for sale, gross unrealized losses 12 months or longer		18
Available for sale, fair value total		253
Available for sale, gross unrealized losses total		18
Asset-backed securities, other
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months		75
Available for sale, gross unrealized losses less than 12 months		0
Available for sale, fair value 12 months or longer		0
Available for sale, gross unrealized losses 12 months or longer		0
Available for sale, fair value total		75
Available for sale, gross unrealized losses total		0
Held to maturity, fair value less than 12 months		18
Held-to-maturity, gross, less than 12 months		0
Held to maturity, fair value 12 months or longer		9
Held to maturity, gross unrealized losses 12 months or longer		0
Held to maturity, fair value total		27
Held to maturity, gross unrealized losses total		0
Asset-backed securities
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	1,289	925
Available for sale, gross unrealized losses less than 12 months	25	1
Available for sale, fair value 12 months or longer	769	4,491
Available for sale, gross unrealized losses 12 months or longer	4	118
Available for sale, fair value total	2,058	5,416
Available for sale, gross unrealized losses total	29	119
Held to maturity, fair value less than 12 months	440	756
Held-to-maturity, gross, less than 12 months	3	9
Held to maturity, fair value 12 months or longer	423	1,244
Held to maturity, gross unrealized losses 12 months or longer	6	21
Held to maturity, fair value total	863	2,000
Held to maturity, gross unrealized losses total	9	30
Non-U.S. debt securities, mortgage-backed securities
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	1,059	442
Available for sale, gross unrealized losses less than 12 months	4	1
Available for sale, fair value 12 months or longer	469	893
Available for sale, gross unrealized losses 12 months or longer	1	5
Available for sale, fair value total	1,528	1,335
Available for sale, gross unrealized losses total	5	6
Held to maturity, fair value less than 12 months	0	54
Held-to-maturity, gross, less than 12 months	0	2
Held to maturity, fair value 12 months or longer	239	330
Held to maturity, gross unrealized losses 12 months or longer	6	13
Held to maturity, fair value total	239	384
Held to maturity, gross unrealized losses total	6	15
Non-U.S. debt securities, asset-backed securities
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months		253
Available for sale, gross unrealized losses less than 12 months		0
Available for sale, fair value 12 months or longer		276
Available for sale, gross unrealized losses 12 months or longer		1
Available for sale, fair value total		529
Available for sale, gross unrealized losses total		1
Held to maturity, fair value less than 12 months		28
Held-to-maturity, gross, less than 12 months		0
Held to maturity, fair value 12 months or longer		35
Held to maturity, gross unrealized losses 12 months or longer		0
Held to maturity, fair value total		63
Held to maturity, gross unrealized losses total		0
Government securities
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	7,629	1,314
Available for sale, gross unrealized losses less than 12 months	48	6
Available for sale, fair value 12 months or longer	68	0
Available for sale, gross unrealized losses 12 months or longer	1	0
Available for sale, fair value total	7,697	1,314
Available for sale, gross unrealized losses total	49	6
Held to maturity, fair value less than 12 months		180
Held-to-maturity, gross, less than 12 months		0
Held to maturity, fair value 12 months or longer		0
Held to maturity, gross unrealized losses 12 months or longer		0
Held to maturity, fair value total		180
Held to maturity, gross unrealized losses total		0
Non-U.S. debt securities, other
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	816	670
Available for sale, gross unrealized losses less than 12 months	4	4
Available for sale, fair value 12 months or longer	289	218
Available for sale, gross unrealized losses 12 months or longer	2	1
Available for sale, fair value total	1,105	888
Available for sale, gross unrealized losses total	6	5
Total non-U.S. debt securities
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	9,504	2,679
Available for sale, gross unrealized losses less than 12 months	56	11
Available for sale, fair value 12 months or longer	826	1,387
Available for sale, gross unrealized losses 12 months or longer	4	7
Available for sale, fair value total	10,330	4,066
Available for sale, gross unrealized losses total	60	18
Held to maturity, fair value less than 12 months	0	262
Held-to-maturity, gross, less than 12 months	0	2
Held to maturity, fair value 12 months or longer	239	365
Held to maturity, gross unrealized losses 12 months or longer	6	13
Held to maturity, fair value total	239	627
Held to maturity, gross unrealized losses total	6	15
State and political subdivisions
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	734	3,390
Available for sale, gross unrealized losses less than 12 months	6	102
Available for sale, fair value 12 months or longer	901	304
Available for sale, gross unrealized losses 12 months or longer	17	10
Available for sale, fair value total	1,635	3,694
Available for sale, gross unrealized losses total	23	112
Collateralized mortgage obligations
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	399	1,259
Available for sale, gross unrealized losses less than 12 months	5	31
Available for sale, fair value 12 months or longer	136	162
Available for sale, gross unrealized losses 12 months or longer	4	4
Available for sale, fair value total	535	1,421
Available for sale, gross unrealized losses total	9	35
Held to maturity, fair value less than 12 months	0	537
Held-to-maturity, gross, less than 12 months	0	4
Held to maturity, fair value 12 months or longer	276	204
Held to maturity, gross unrealized losses 12 months or longer	6	7
Held to maturity, fair value total	276	741
Held to maturity, gross unrealized losses total	6	11
Other U.S. debt securities
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	1,007	944
Available for sale, gross unrealized losses less than 12 months	8	24
Available for sale, fair value 12 months or longer	345	157
Available for sale, gross unrealized losses 12 months or longer	7	6
Available for sale, fair value total	1,352	1,101
Available for sale, gross unrealized losses total	15	30
U.S. equity securities
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	0	8
Available for sale, gross unrealized losses less than 12 months	0	0
Available for sale, fair value 12 months or longer	6	5
Available for sale, gross unrealized losses 12 months or longer	2	3
Available for sale, fair value total	6	13
Available for sale, gross unrealized losses total	$ 2	$ 3