Investment Securities - Schedule of Gross Pre-Tax Unrealized Losses on Investment Securities (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	$ 17,950	$ 18,094
Available for sale, gross unrealized losses less than 12 months	235	131
Available for sale, fair value 12 months or longer	4,845	6,391
Available for sale, gross unrealized losses 12 months or longer	174	137
Available for sale, fair value total	22,795	24,485
Available for sale, gross unrealized losses total	409	268
Held to maturity, fair value less than 12 months	24,912	21,664
Held-to-maturity, gross, less than 12 months	469	150
Held to maturity, fair value 12 months or longer	10,399	9,373
Held to maturity, gross unrealized losses 12 months or longer	382	239
Held to maturity, fair value total	35,311	31,037
Held to maturity, gross unrealized losses total	851	389
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	8	0
Available for sale, gross unrealized losses less than 12 months	0	0
Available for sale, fair value 12 months or longer	18	67
Available for sale, gross unrealized losses 12 months or longer	0	1
Available for sale, fair value total	26	67
Available for sale, gross unrealized losses total	0	1
Held to maturity, fair value less than 12 months	13,137	14,439
Held-to-maturity, gross, less than 12 months	216	109
Held to maturity, fair value 12 months or longer	3,507	2,447
Held to maturity, gross unrealized losses 12 months or longer	43	34
Held to maturity, fair value total	16,644	16,886
Held to maturity, gross unrealized losses total	259	143
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	5,595	5,161
Available for sale, gross unrealized losses less than 12 months	108	31
Available for sale, fair value 12 months or longer	3,140	3,341
Available for sale, gross unrealized losses 12 months or longer	136	98
Available for sale, fair value total	8,735	8,502
Available for sale, gross unrealized losses total	244	129
Held to maturity, fair value less than 12 months	11,370	6,785
Held-to-maturity, gross, less than 12 months	252	38
Held to maturity, fair value 12 months or longer	5,820	5,988
Held to maturity, gross unrealized losses 12 months or longer	319	187
Held to maturity, fair value total	17,190	12,773
Held to maturity, gross unrealized losses total	571	225
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	5,603	5,161
Available for sale, gross unrealized losses less than 12 months	108	31
Available for sale, fair value 12 months or longer	3,158	3,408
Available for sale, gross unrealized losses 12 months or longer	136	99
Available for sale, fair value total	8,761	8,569
Available for sale, gross unrealized losses total	244	130
Held to maturity, fair value less than 12 months	24,507	21,224
Held-to-maturity, gross, less than 12 months	468	147
Held to maturity, fair value 12 months or longer	9,327	8,435
Held to maturity, gross unrealized losses 12 months or longer	362	221
Held to maturity, fair value total	33,834	29,659
Held to maturity, gross unrealized losses total	830	368
Asset-backed securities, student loans
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	315	0
Available for sale, gross unrealized losses less than 12 months	0	0
Available for sale, fair value 12 months or longer	251	769
Available for sale, gross unrealized losses 12 months or longer	2	4
Available for sale, fair value total	566	769
Available for sale, gross unrealized losses total	2	4
Held to maturity, fair value less than 12 months	131	440
Held-to-maturity, gross, less than 12 months	1	3
Held to maturity, fair value 12 months or longer	572	423
Held to maturity, gross unrealized losses 12 months or longer	8	6
Held to maturity, fair value total	703	863
Held to maturity, gross unrealized losses total	9	9
Asset-backed securities, credit cards
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	1,280	1,289
Available for sale, gross unrealized losses less than 12 months	31	25
Available for sale, fair value 12 months or longer	0	0
Available for sale, gross unrealized losses 12 months or longer	0	0
Available for sale, fair value total	1,280	1,289
Available for sale, gross unrealized losses total	31	25
Asset-backed securities, other
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Held to maturity, fair value less than 12 months	0
Held-to-maturity, gross, less than 12 months	0
Held to maturity, fair value 12 months or longer	1
Held to maturity, gross unrealized losses 12 months or longer	0
Held to maturity, fair value total	1
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	1,615	1,289
Available for sale, gross unrealized losses less than 12 months	31	25
Available for sale, fair value 12 months or longer	251	769
Available for sale, gross unrealized losses 12 months or longer	2	4
Available for sale, fair value total	1,866	2,058
Available for sale, gross unrealized losses total	33	29
Held to maturity, fair value less than 12 months	131	440
Held-to-maturity, gross, less than 12 months	1	3
Held to maturity, fair value 12 months or longer	573	423
Held to maturity, gross unrealized losses 12 months or longer	8	6
Held to maturity, fair value total	704	863
Held to maturity, gross unrealized losses total	9	9
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	379	1,059
Available for sale, gross unrealized losses less than 12 months	1	4
Available for sale, fair value 12 months or longer	161	469
Available for sale, gross unrealized losses 12 months or longer	0	1
Available for sale, fair value total	540	1,528
Available for sale, gross unrealized losses total	1	5
Held to maturity, fair value less than 12 months	26	0
Held-to-maturity, gross, less than 12 months	0	0
Held to maturity, fair value 12 months or longer	222	239
Held to maturity, gross unrealized losses 12 months or longer	5	6
Held to maturity, fair value total	248	239
Held to maturity, gross unrealized losses total	5	6
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	130
Available for sale, gross unrealized losses less than 12 months	0
Available for sale, fair value 12 months or longer	33
Available for sale, gross unrealized losses 12 months or longer	0
Available for sale, fair value total	163
Available for sale, gross unrealized losses total	0
Held to maturity, fair value less than 12 months	0
Held-to-maturity, gross, less than 12 months	0
Held to maturity, fair value 12 months or longer	13
Held to maturity, gross unrealized losses 12 months or longer	0
Held to maturity, fair value total	13
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	6,392	7,629
Available for sale, gross unrealized losses less than 12 months	31	48
Available for sale, fair value 12 months or longer	69	68
Available for sale, gross unrealized losses 12 months or longer	1	1
Available for sale, fair value total	6,461	7,697
Available for sale, gross unrealized losses total	32	49
Held to maturity, fair value less than 12 months	242
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	242
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	931	816
Available for sale, gross unrealized losses less than 12 months	12	4
Available for sale, fair value 12 months or longer	260	289
Available for sale, gross unrealized losses 12 months or longer	3	2
Available for sale, fair value total	1,191	1,105
Available for sale, gross unrealized losses total	15	6
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	7,832	9,504
Available for sale, gross unrealized losses less than 12 months	44	56
Available for sale, fair value 12 months or longer	523	826
Available for sale, gross unrealized losses 12 months or longer	4	4
Available for sale, fair value total	8,355	10,330
Available for sale, gross unrealized losses total	48	60
Held to maturity, fair value less than 12 months	268	0
Held-to-maturity, gross, less than 12 months	0	0
Held to maturity, fair value 12 months or longer	235	239
Held to maturity, gross unrealized losses 12 months or longer	5	6
Held to maturity, fair value total	503	239
Held to maturity, gross unrealized losses total	5	6
State and political subdivisions
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	1,306	734
Available for sale, gross unrealized losses less than 12 months	22	6
Available for sale, fair value 12 months or longer	546	901
Available for sale, gross unrealized losses 12 months or longer	22	17
Available for sale, fair value total	1,852	1,635
Available for sale, gross unrealized losses total	44	23
Collateralized mortgage obligations
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	237	399
Available for sale, gross unrealized losses less than 12 months	3	5
Available for sale, fair value 12 months or longer	75	136
Available for sale, gross unrealized losses 12 months or longer	2	4
Available for sale, fair value total	312	535
Available for sale, gross unrealized losses total	5	9
Held to maturity, fair value less than 12 months	6	0
Held-to-maturity, gross, less than 12 months	0	0
Held to maturity, fair value 12 months or longer	264	276
Held to maturity, gross unrealized losses 12 months or longer	7	6
Held to maturity, fair value total	270	276
Held to maturity, gross unrealized losses total	7	6
Other U.S. debt securities
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	1,357	1,007
Available for sale, gross unrealized losses less than 12 months	27	8
Available for sale, fair value 12 months or longer	292	345
Available for sale, gross unrealized losses 12 months or longer	8	7
Available for sale, fair value total	1,649	1,352
Available for sale, gross unrealized losses total	$ 35	15
U.S. equity securities
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months		0
Available for sale, gross unrealized losses less than 12 months		0
Available for sale, fair value 12 months or longer		6
Available for sale, gross unrealized losses 12 months or longer		2
Available for sale, fair value total		6
Available for sale, gross unrealized losses total		$ 2