FAIR VALUE MEASUREMENT - Items Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Assets, Fair Value Disclosure [Abstract]
Federal funds sold and securities borrowed or purchased under agreements to resell, selected portfolios of securities purchased under agreements to resell, Netting	$ (87,424)	$ (73,476)
Trading account assets	256,117	252,790
Netting of cash collateral received	(38,608)	(38,532)
Investments	358,607	352,290
Loans	3,223	4,374
Liabilities, Fair Value Disclosure [Abstract]
Federal funds purchased and securities loaned or sold under agreements to repurchase, selected portfolios of securities sold under agreements to repurchase, Netting	(87,424)	(73,476)
Netting of cash collateral paid	(29,911)	(35,666)
Fair Value Measured at Net Asset Value Per Share
Liabilities, Fair Value Disclosure [Abstract]
Alternative investment funds, fair value	207	404
Fair Value Measured at Net Asset Value Per Share | Accounting Standards Update 2015-07
Liabilities, Fair Value Disclosure [Abstract]
Alternative investment funds, fair value	200	400
Recurring
Assets, Fair Value Disclosure [Abstract]
Federal funds sold and securities borrowed or purchased under agreements to resell	214,685	188,587
Federal funds sold and securities borrowed or purchased under agreements to resell, selected portfolios of securities purchased under agreements to resell, Netting	(66,984)	(55,638)
Federal funds sold and securities borrowed or purchased under agreements to resell, selected portfolios of securities purchased under agreements to resell	147,701	132,949
Investments	288,940	291,716
Loans	3,223	4,374
Mortgage servicing rights (MSRs)	584	558
Assets before netting	1,134,034	1,101,517
Total assets, Netting	(416,681)	(400,571)
Total assets	717,353	700,946
Liabilities, Fair Value Disclosure [Abstract]
Interest-bearing deposits	1,475	1,465
Federal funds purchased and securities loaned or sold under agreements to repurchase, selected portfolios of securities sold under agreements to repurchase, Gross	111,494	96,276
Federal funds purchased and securities loaned or sold under agreements to repurchase, selected portfolios of securities sold under agreements to repurchase, Netting	(66,984)	(55,638)
Federal funds purchased and securities loaned or sold under agreements to repurchase, selected portfolios of securities sold under agreements to repurchase	44,510	40,638
Securities sold, not yet purchased	90,822	76,171
Trading liabilities	92,369	77,585
Short-term borrowings	4,483	4,627
Long-term debt	38,229	31,392
Total liabilities, Gross	656,892	614,958
Total liabilities, Netting	(407,984)	(397,705)
Total liabilities	248,908	217,253
Recurring | Trading derivatives liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	379,030	375,344
Cash collateral received	13,906	14,308
Total trading derivatives and cash collateral, liability	392,936	389,652
Netting agreements	(311,089)	(306,401)
Netting of cash collateral paid	(29,911)	(35,666)
Netting, Liabilities, total of netting agreements and cash collateral received	(341,000)	(342,067)
Total derivative liabilities	51,936	47,585
Gross cash collateral received	52,514	52,840
Recurring | Trading derivatives liabilities | Interest rate contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	154,827	184,639
Recurring | Trading derivatives liabilities | Foreign exchange contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	159,355	120,772
Recurring | Trading derivatives liabilities | Equity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	33,808	31,373
Recurring | Trading derivatives liabilities | Commodity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	20,689	17,027
Recurring | Trading derivatives liabilities | Credit derivatives
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	10,351	21,533
Recurring | Non-trading derivatives and other financial liabilities
Liabilities, Fair Value Disclosure [Abstract]
Netting, Liabilities, total of netting agreements and cash collateral received	0	0
Non-trading derivatives and other financial liabilities measured on a recurring basis, gross	15,906	13,961
Total other assets and cash collateral, gross	15,906	13,961
Recurring | Trading account liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	1,547	1,414
Recurring | Mortgage-backed securities - U.S. agency-sponsored
Assets, Fair Value Disclosure [Abstract]
Trading account assets	24,246	22,964
Investments	43,020	41,741
Recurring | Mortgage-backed securities - Residential
Assets, Fair Value Disclosure [Abstract]
Trading account assets	977	813
Investments	1,313	2,884
Recurring | Mortgage-backed securities - Commercial
Assets, Fair Value Disclosure [Abstract]
Trading account assets	1,400	1,366
Investments	172	332
Recurring | Mortgage-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	26,623	25,143
Investments	44,505	44,957
Recurring | U.S. Treasury and federal agency securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	31,242	21,137
Investments	117,222	118,146
Recurring | State and municipal securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	3,982	4,700
Investments	9,206	8,765
Recurring | Foreign government
Assets, Fair Value Disclosure [Abstract]
Trading account assets	64,519	60,206
Investments	100,691	100,533
Recurring | Corporate
Assets, Fair Value Disclosure [Abstract]
Trading account assets	15,896	15,705
Investments	11,599	14,109
Recurring | Equity securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	43,803	60,219
Investments	220	189
Recurring | Asset-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	2,913	2,788
Investments	843	3,918
Recurring | Other debt securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	13,019	11,723
Investments	3,972	297
Recurring | Non-marketable equity securities
Assets, Fair Value Disclosure [Abstract]
Investments	682	802
Recurring | Trading securities (excluding trading account derivatives)
Assets, Fair Value Disclosure [Abstract]
Trading account assets	201,997	201,621
Recurring | Trading account assets
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	392,299	388,561
Gross cash collateral paid	11,518	7,541
Trading derivative, asset, gross net cash collateral paid	403,817	396,102
Less: Netting agreements to assets	(311,089)	(306,401)
Netting of cash collateral received	(38,608)	(38,532)
Netting, Assets, total of netting agreements and cash collateral received	(349,697)	(344,933)
Trading derivatives	54,120	51,169
Liabilities, Fair Value Disclosure [Abstract]
Gross cash collateral paid	41,429	43,207
Recurring | Trading account assets | Interest rate contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	171,632	204,987
Recurring | Trading account assets | Foreign exchange contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	162,454	121,959
Recurring | Trading account assets | Equity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	29,893	26,978
Recurring | Trading account assets | Commodity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	17,555	14,103
Recurring | Trading account assets | Credit derivatives
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	10,765	20,534
Recurring | Non-trading derivatives and other financial assets
Assets, Fair Value Disclosure [Abstract]
Netting, Assets, total of netting agreements and cash collateral received	0	0
Total other assets and cash collateral, gross	20,788	18,559
Other assets	20,788	18,559
Recurring | Level 1
Assets, Fair Value Disclosure [Abstract]
Federal funds sold and securities borrowed or purchased under agreements to resell	0	0
Investments	170,445	165,507
Loans	0	0
Mortgage servicing rights (MSRs)	0	0
Assets before netting	$ 294,151	$ 292,700
Total as a percentage of gross assets	26.20%	26.80%
Liabilities, Fair Value Disclosure [Abstract]
Interest-bearing deposits	$ 0	$ 0
Federal funds purchased and securities loaned or sold under agreements to repurchase, selected portfolios of securities sold under agreements to repurchase, Gross	0	0
Securities sold, not yet purchased	78,872	65,843
Trading liabilities	78,872	65,843
Short-term borrowings	0	0
Long-term debt	0	0
Total liabilities, Gross	$ 95,133	$ 82,437
Total as a percentage of gross liabilities	14.80%	13.70%
Recurring | Level 1 | Trading derivatives liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	$ 422	$ 2,691
Recurring | Level 1 | Trading derivatives liabilities | Interest rate contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	71	137
Recurring | Level 1 | Trading derivatives liabilities | Foreign exchange contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	0	9
Recurring | Level 1 | Trading derivatives liabilities | Equity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	351	2,430
Recurring | Level 1 | Trading derivatives liabilities | Commodity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	0	115
Recurring | Level 1 | Trading derivatives liabilities | Credit derivatives
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	0	0
Recurring | Level 1 | Non-trading derivatives and other financial liabilities
Liabilities, Fair Value Disclosure [Abstract]
Other liabilities, gross	15,839	13,903
Recurring | Level 1 | Trading account liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	0	0
Recurring | Level 1 | Mortgage-backed securities - U.S. agency-sponsored
Assets, Fair Value Disclosure [Abstract]
Trading account assets	0	0
Investments	0	0
Recurring | Level 1 | Mortgage-backed securities - Residential
Assets, Fair Value Disclosure [Abstract]
Trading account assets	0	0
Investments	0	0
Recurring | Level 1 | Mortgage-backed securities - Commercial
Assets, Fair Value Disclosure [Abstract]
Trading account assets	0	0
Investments	0	0
Recurring | Level 1 | Mortgage-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	0	0
Investments	0	0
Recurring | Level 1 | U.S. Treasury and federal agency securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	26,439	17,524
Investments	107,577	106,964
Recurring | Level 1 | State and municipal securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	0	0
Investments	0	0
Recurring | Level 1 | Foreign government
Assets, Fair Value Disclosure [Abstract]
Trading account assets	43,309	39,347
Investments	58,252	56,456
Recurring | Level 1 | Corporate
Assets, Fair Value Disclosure [Abstract]
Trading account assets	1,026	301
Investments	4,410	1,911
Recurring | Level 1 | Equity securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	36,342	53,305
Investments	206	176
Recurring | Level 1 | Asset-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	0	0
Investments	0	0
Recurring | Level 1 | Other debt securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	3	3
Investments	0	0
Recurring | Level 1 | Non-marketable equity securities
Assets, Fair Value Disclosure [Abstract]
Investments	0	0
Recurring | Level 1 | Trading securities (excluding trading account derivatives)
Assets, Fair Value Disclosure [Abstract]
Trading account assets	107,119	110,480
Recurring | Level 1 | Trading account assets
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	748	2,810
Recurring | Level 1 | Trading account assets | Interest rate contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	101	145
Recurring | Level 1 | Trading account assets | Foreign exchange contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	0	19
Recurring | Level 1 | Trading account assets | Equity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	647	2,364
Recurring | Level 1 | Trading account assets | Commodity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	0	282
Recurring | Level 1 | Trading account assets | Credit derivatives
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	0	0
Recurring | Level 1 | Non-trading derivatives and other financial assets
Assets, Fair Value Disclosure [Abstract]
Other assets, gross	15,839	13,903
Recurring | Level 2
Assets, Fair Value Disclosure [Abstract]
Federal funds sold and securities borrowed or purchased under agreements to resell	214,570	188,571
Investments	116,758	123,772
Loans	2,946	3,824
Mortgage servicing rights (MSRs)	0	0
Assets before netting	$ 818,051	$ 790,217
Total as a percentage of gross assets	72.90%	72.20%
Liabilities, Fair Value Disclosure [Abstract]
Interest-bearing deposits	$ 980	$ 1,179
Federal funds purchased and securities loaned or sold under agreements to repurchase, selected portfolios of securities sold under agreements to repurchase, Gross	110,511	95,550
Securities sold, not yet purchased	11,364	10,306
Trading liabilities	12,911	11,715
Short-term borrowings	4,446	4,609
Long-term debt	25,659	18,310
Total liabilities, Gross	$ 528,228	$ 494,879
Total as a percentage of gross liabilities	82.10%	82.40%
Recurring | Level 2 | Trading derivatives liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	$ 373,654	$ 363,466
Recurring | Level 2 | Trading derivatives liabilities | Interest rate contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	152,931	182,372
Recurring | Level 2 | Trading derivatives liabilities | Foreign exchange contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	159,003	120,316
Recurring | Level 2 | Trading derivatives liabilities | Equity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	32,330	26,472
Recurring | Level 2 | Trading derivatives liabilities | Commodity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	19,904	14,482
Recurring | Level 2 | Trading derivatives liabilities | Credit derivatives
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	9,486	19,824
Recurring | Level 2 | Non-trading derivatives and other financial liabilities
Liabilities, Fair Value Disclosure [Abstract]
Other liabilities, gross	67	50
Recurring | Level 2 | Trading account liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	1,547	1,409
Recurring | Level 2 | Mortgage-backed securities - U.S. agency-sponsored
Assets, Fair Value Disclosure [Abstract]
Trading account assets	24,090	22,801
Investments	42,988	41,717
Recurring | Level 2 | Mortgage-backed securities - Residential
Assets, Fair Value Disclosure [Abstract]
Trading account assets	709	649
Investments	1,313	2,884
Recurring | Level 2 | Mortgage-backed securities - Commercial
Assets, Fair Value Disclosure [Abstract]
Trading account assets	1,323	1,309
Investments	172	329
Recurring | Level 2 | Mortgage-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	26,122	24,759
Investments	44,473	44,930
Recurring | Level 2 | U.S. Treasury and federal agency securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	4,802	3,613
Investments	9,645	11,182
Recurring | Level 2 | State and municipal securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	3,782	4,426
Investments	8,498	8,028
Recurring | Level 2 | Foreign government
Assets, Fair Value Disclosure [Abstract]
Trading account assets	21,179	20,843
Investments	42,371	43,985
Recurring | Level 2 | Corporate
Assets, Fair Value Disclosure [Abstract]
Trading account assets	14,510	15,129
Investments	7,033	12,127
Recurring | Level 2 | Equity securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	7,308	6,794
Investments	14	11
Recurring | Level 2 | Asset-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	1,429	1,198
Investments	656	3,091
Recurring | Level 2 | Other debt securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	12,198	11,105
Investments	3,972	297
Recurring | Level 2 | Non-marketable equity securities
Assets, Fair Value Disclosure [Abstract]
Investments	96	121
Recurring | Level 2 | Trading securities (excluding trading account derivatives)
Assets, Fair Value Disclosure [Abstract]
Trading account assets	91,330	87,867
Recurring | Level 2 | Trading account assets
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	387,498	381,543
Recurring | Level 2 | Trading account assets | Interest rate contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	169,860	203,134
Recurring | Level 2 | Trading account assets | Foreign exchange contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	162,108	121,363
Recurring | Level 2 | Trading account assets | Equity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	28,903	24,170
Recurring | Level 2 | Trading account assets | Commodity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	16,788	13,252
Recurring | Level 2 | Trading account assets | Credit derivatives
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	9,839	19,624
Recurring | Level 2 | Non-trading derivatives and other financial assets
Assets, Fair Value Disclosure [Abstract]
Other assets, gross	4,949	4,640
Recurring | Level 3
Assets, Fair Value Disclosure [Abstract]
Federal funds sold and securities borrowed or purchased under agreements to resell	115	16
Investments	1,737	2,437
Loans	277	550
Mortgage servicing rights (MSRs)	584	558
Assets before netting	$ 10,314	$ 11,059
Total as a percentage of gross assets	0.90%	1.00%
Liabilities, Fair Value Disclosure [Abstract]
Interest-bearing deposits	$ 495	$ 286
Federal funds purchased and securities loaned or sold under agreements to repurchase, selected portfolios of securities sold under agreements to repurchase, Gross	983	726
Securities sold, not yet purchased	586	22
Trading liabilities	586	27
Short-term borrowings	37	18
Long-term debt	12,570	13,082
Total liabilities, Gross	$ 19,625	$ 23,334
Total as a percentage of gross liabilities	3.10%	3.90%
Recurring | Level 3 | Trading derivatives liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	$ 4,954	$ 9,187
Recurring | Level 3 | Trading derivatives liabilities | Interest rate contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	1,825	2,130
Recurring | Level 3 | Trading derivatives liabilities | Foreign exchange contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	352	447
Recurring | Level 3 | Trading derivatives liabilities | Equity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	1,127	2,471
Recurring | Level 3 | Trading derivatives liabilities | Commodity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	785	2,430
Recurring | Level 3 | Trading derivatives liabilities | Credit derivatives
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	865	1,709
Recurring | Level 3 | Non-trading derivatives and other financial liabilities
Liabilities, Fair Value Disclosure [Abstract]
Other liabilities, gross	0	8
Recurring | Level 3 | Trading account liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	0	5
Recurring | Level 3 | Mortgage-backed securities - U.S. agency-sponsored
Assets, Fair Value Disclosure [Abstract]
Trading account assets	156	163
Investments	32	24
Recurring | Level 3 | Mortgage-backed securities - Residential
Assets, Fair Value Disclosure [Abstract]
Trading account assets	268	164
Investments	0	0
Recurring | Level 3 | Mortgage-backed securities - Commercial
Assets, Fair Value Disclosure [Abstract]
Trading account assets	77	57
Investments	0	3
Recurring | Level 3 | Mortgage-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	501	384
Investments	32	27
Recurring | Level 3 | U.S. Treasury and federal agency securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	1	0
Investments	0	0
Recurring | Level 3 | State and municipal securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	200	274
Investments	708	737
Recurring | Level 3 | Foreign government
Assets, Fair Value Disclosure [Abstract]
Trading account assets	31	16
Investments	68	92
Recurring | Level 3 | Corporate
Assets, Fair Value Disclosure [Abstract]
Trading account assets	360	275
Investments	156	71
Recurring | Level 3 | Equity securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	153	120
Investments	0	2
Recurring | Level 3 | Asset-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	1,484	1,590
Investments	187	827
Recurring | Level 3 | Other debt securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	818	615
Investments	0	0
Recurring | Level 3 | Non-marketable equity securities
Assets, Fair Value Disclosure [Abstract]
Investments	586	681
Recurring | Level 3 | Trading securities (excluding trading account derivatives)
Assets, Fair Value Disclosure [Abstract]
Trading account assets	3,548	3,274
Recurring | Level 3 | Trading account assets
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	4,053	4,208
Recurring | Level 3 | Trading account assets | Interest rate contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	1,671	1,708
Recurring | Level 3 | Trading account assets | Foreign exchange contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	346	577
Recurring | Level 3 | Trading account assets | Equity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	343	444
Recurring | Level 3 | Trading account assets | Commodity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	767	569
Recurring | Level 3 | Trading account assets | Credit derivatives
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	926	910
Recurring | Level 3 | Non-trading derivatives and other financial assets
Assets, Fair Value Disclosure [Abstract]
Other assets, gross	$ 0	$ 16