FAIR VALUE MEASUREMENT - Items Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Assets, Fair Value Disclosure [Abstract]
Federal funds sold and securities borrowed and purchased under agreements to resell, Netting	$ (94,176)	$ (157,370)
Trading account assets	370,950	375,079
Netting of cash collateral received	(43,554)	(63,915)
Investments	487,063	447,359
Loans held at fair value	7,707	6,854
Liabilities, Fair Value Disclosure [Abstract]
Federal funds purchased and securities loaned and sold under agreements to repurchase, Netting	(94,176)	(157,370)
Netting of cash collateral paid	(48,079)	(45,628)
Fair value
Liabilities, Fair Value Disclosure [Abstract]
Investments measured at net asset value excluded from Level 3	147	152
Recurring
Assets, Fair Value Disclosure [Abstract]
Federal funds sold and securities borrowed and purchased under agreements to resell	275,524	335,393
Federal funds sold and securities borrowed and purchased under agreements to resell, Netting	(87,648)	(150,189)
Federal funds sold and securities borrowed and purchased under agreements to resell	187,876	185,204
Investments	303,623	335,998
Loans held at fair value	7,707	6,854
Mortgage servicing rights	419	336
Assets before netting	1,305,907	1,497,067
Netting, Assets, total of netting agreements and cash collateral received	(424,249)	(578,983)
Total assets	881,658	918,084
Liabilities, Fair Value Disclosure [Abstract]
Interest-bearing deposits	2,749	1,958
Federal funds purchased and securities loaned and sold under agreements to repurchase, Gross	157,576	157,275
Federal funds purchased and securities loaned and sold under agreements to repurchase, Netting	(80,871)	(97,069)
Federal funds purchased and securities loaned and sold under agreements to repurchase	76,705	60,206
Securities sold, not yet purchased	113,787	100,044
Trading liabilities	113,801	100,070
Short-term borrowings	7,358	4,683
Long-term debt	76,375	67,063
Total liabilities, Gross	763,169	816,348
Total liabilities, Netting	(421,997)	(507,576)
Total liabilities	341,172	308,772
Recurring | Trading derivatives
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	384,066	470,268
Cash collateral received	17,965	8,196
Total trading derivatives and cash collateral, liability	402,031	478,464
Netting agreements	(293,047)	(364,879)
Netting of cash collateral paid	(48,079)	(45,628)
Netting, Liabilities, total of netting agreements and cash collateral received	(341,126)	(410,507)
Total derivative liabilities	60,905	67,957
Cash collateral received, gross	61,519	72,111
Recurring | Trading derivatives | Interest rate contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	168,741	222,411
Recurring | Trading derivatives | Foreign exchange contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	116,735	156,066
Recurring | Trading derivatives | Equity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	59,927	63,569
Recurring | Trading derivatives | Commodity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	28,186	18,093
Recurring | Trading derivatives | Credit derivatives
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	10,477	10,129
Recurring | Non-trading derivatives and other financial liabilities measured on a recurring basis
Liabilities, Fair Value Disclosure [Abstract]
Netting of cash collateral paid		0
Netting, Liabilities, total of netting agreements and cash collateral received
Non-trading derivatives and other financial liabilities measured on a recurring basis, gross	3,279	6,835
Total other assets and cash collateral, gross	3,279	6,835
Recurring | Trading account liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	14	26
Recurring | Mortgage-backed securities - U.S. government-sponsored agency guaranteed
Assets, Fair Value Disclosure [Abstract]
Trading account assets	32,374	42,930
Investments	39,310	43,918
Recurring | Residential mortgages
Assets, Fair Value Disclosure [Abstract]
Trading account assets	463	731
Investments	404	571
Recurring | Commercial and other
Assets, Fair Value Disclosure [Abstract]
Trading account assets	987	1,029
Investments	43	50
Recurring | Mortgage-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	33,824	44,690
Investments	39,757	44,539
Recurring | U.S. Treasury and federal agency securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	63,495	66,798
Investments	126,147	146,204
Recurring | State and municipal
Assets, Fair Value Disclosure [Abstract]
Trading account assets	1,610	1,318
Investments	3,079	3,719
Recurring | Foreign government
Assets, Fair Value Disclosure [Abstract]
Trading account assets	91,072	83,389
Investments	120,217	124,968
Recurring | Corporate
Assets, Fair Value Disclosure [Abstract]
Trading account assets	21,602	20,822
Investments	7,903	10,500
Recurring | Equity securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	68,326	66,479
Investments	195	515
Recurring | Asset-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	1,708	2,382
Investments	211	278
Recurring | Other debt securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	15,698	12,240
Investments	5,663	4,876
Recurring | Non-marketable equity securities measured using the measurement alternative
Assets, Fair Value Disclosure [Abstract]
Investments	451	399
Recurring | Trading non-derivative assets
Assets, Fair Value Disclosure [Abstract]
Trading account assets	297,335	298,118
Recurring | Trading account derivatives
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	388,112	472,977
Cash collateral paid	22,104	32,778
Trading derivative, asset, gross net cash collateral paid	410,216	505,755
Netting agreements	(293,047)	(364,879)
Netting of cash collateral received	(43,554)	(63,915)
Total trading derivatives, netting	(336,601)	(428,794)
Trading derivatives	73,615	76,961
Liabilities, Fair Value Disclosure [Abstract]
Cash collateral paid, gross	70,183	78,406
Recurring | Trading account derivatives | Interest rate contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	185,977	241,461
Recurring | Trading account derivatives | Foreign exchange contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	118,571	156,670
Recurring | Trading account derivatives | Equity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	48,515	50,519
Recurring | Trading account derivatives | Commodity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	25,197	14,538
Recurring | Trading account derivatives | Credit derivatives
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	9,852	9,789
Recurring | Non-trading derivatives and other financial assets measured on a recurring basis
Assets, Fair Value Disclosure [Abstract]
Total trading derivatives, netting	0	0
Total other assets and cash collateral, gross	11,083	14,613
Other assets	11,083	14,613
Recurring | Level 1
Assets, Fair Value Disclosure [Abstract]
Federal funds sold and securities borrowed and purchased under agreements to resell	0	0
Investments	197,191	229,701
Loans held at fair value	0	0
Mortgage servicing rights	0	0
Assets before netting	$ 379,311	$ 424,489
Total as a percentage of gross assets	29.50%	29.00%
Liabilities, Fair Value Disclosure [Abstract]
Interest-bearing deposits	$ 0	$ 0
Federal funds purchased and securities loaned and sold under agreements to repurchase, Gross	0	0
Securities sold, not yet purchased	91,879	85,353
Trading liabilities	91,879	85,353
Short-term borrowings	0	0
Long-term debt	0	0
Total liabilities, Gross	$ 95,075	$ 92,196
Total as a percentage of gross liabilities	12.80%	11.40%
Recurring | Level 1 | Trading derivatives
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	$ 44	$ 81
Recurring | Level 1 | Trading derivatives | Interest rate contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	42	25
Recurring | Level 1 | Trading derivatives | Foreign exchange contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	2	3
Recurring | Level 1 | Trading derivatives | Equity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	0	53
Recurring | Level 1 | Trading derivatives | Commodity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	0	0
Recurring | Level 1 | Trading derivatives | Credit derivatives
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	0	0
Recurring | Level 1 | Non-trading derivatives and other financial liabilities measured on a recurring basis
Liabilities, Fair Value Disclosure [Abstract]
Other liabilities, gross	3,152	6,762
Recurring | Level 1 | Trading account liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	0	0
Recurring | Level 1 | Mortgage-backed securities - U.S. government-sponsored agency guaranteed
Assets, Fair Value Disclosure [Abstract]
Trading account assets	0	0
Investments	0	0
Recurring | Level 1 | Residential mortgages
Assets, Fair Value Disclosure [Abstract]
Trading account assets	0	0
Investments	0	0
Recurring | Level 1 | Commercial and other
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	60,491	64,529
Investments	126,147	146,032
Recurring | Level 1 | State and municipal
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	60,726	68,195
Investments	66,557	77,056
Recurring | Level 1 | Corporate
Assets, Fair Value Disclosure [Abstract]
Trading account assets	1,939	1,607
Investments	4,358	6,326
Recurring | Level 1 | Equity securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	55,141	54,117
Investments	129	287
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	305	0
Investments	0	0
Recurring | Level 1 | Non-marketable equity securities measured using the measurement alternative
Assets, Fair Value Disclosure [Abstract]
Investments	0	0
Recurring | Level 1 | Trading non-derivative assets
Assets, Fair Value Disclosure [Abstract]
Trading account assets	178,602	188,448
Recurring | Level 1 | Trading account derivatives
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	61	110
Recurring | Level 1 | Trading account derivatives | Interest rate contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	61	42
Recurring | Level 1 | Trading account derivatives | Foreign exchange contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	0	2
Recurring | Level 1 | Trading account derivatives | Equity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	0	66
Recurring | Level 1 | Trading account derivatives | Commodity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	0	0
Recurring | Level 1 | Trading account derivatives | Credit derivatives
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	0	0
Recurring | Level 1 | Non-trading derivatives and other financial assets measured on a recurring basis
Assets, Fair Value Disclosure [Abstract]
Other assets, gross	3,457	6,230
Recurring | Level 2
Assets, Fair Value Disclosure [Abstract]
Federal funds sold and securities borrowed and purchased under agreements to resell	275,313	335,073
Investments	104,186	104,755
Loans held at fair value	7,278	4,869
Mortgage servicing rights	0	0
Assets before netting	$ 891,525	$ 1,023,665
Total as a percentage of gross assets	69.40%	69.90%
Liabilities, Fair Value Disclosure [Abstract]
Interest-bearing deposits	$ 2,595	$ 1,752
Federal funds purchased and securities loaned and sold under agreements to repurchase, Gross	157,088	156,644
Securities sold, not yet purchased	21,740	14,477
Trading liabilities	21,753	14,477
Short-term borrowings	7,317	4,464
Long-term debt	51,307	41,853
Total liabilities, Gross	$ 616,715	$ 679,937
Total as a percentage of gross liabilities	82.80%	84.10%
Recurring | Level 2 | Trading derivatives
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	$ 376,532	$ 460,675
Recurring | Level 2 | Trading derivatives | Interest rate contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	167,686	220,607
Recurring | Level 2 | Trading derivatives | Foreign exchange contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	115,996	155,441
Recurring | Level 2 | Trading derivatives | Equity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	55,835	58,212
Recurring | Level 2 | Trading derivatives | Commodity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	27,437	17,393
Recurring | Level 2 | Trading derivatives | Credit derivatives
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	9,578	9,022
Recurring | Level 2 | Non-trading derivatives and other financial liabilities measured on a recurring basis
Liabilities, Fair Value Disclosure [Abstract]
Other liabilities, gross	123	72
Recurring | Level 2 | Trading account liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	13	0
Recurring | Level 2 | Mortgage-backed securities - U.S. government-sponsored agency guaranteed
Assets, Fair Value Disclosure [Abstract]
Trading account assets	31,998	42,903
Investments	39,258	43,888
Recurring | Level 2 | Residential mortgages
Assets, Fair Value Disclosure [Abstract]
Trading account assets	368	391
Investments	404	571
Recurring | Level 2 | Commercial and other
Assets, Fair Value Disclosure [Abstract]
Trading account assets	900	893
Investments	43	50
Recurring | Level 2 | Mortgage-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	33,266	44,187
Investments	39,705	44,509
Recurring | Level 2 | U.S. Treasury and federal agency securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	3,004	2,269
Investments	0	172
Recurring | Level 2 | State and municipal
Assets, Fair Value Disclosure [Abstract]
Trading account assets	1,540	1,224
Investments	2,331	2,885
Recurring | Level 2 | Foreign government
Assets, Fair Value Disclosure [Abstract]
Trading account assets	30,205	15,143
Investments	52,703	47,644
Recurring | Level 2 | Corporate
Assets, Fair Value Disclosure [Abstract]
Trading account assets	18,840	18,840
Investments	3,441	4,114
Recurring | Level 2 | Equity securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	13,038	12,289
Investments	66	228
Recurring | Level 2 | Asset-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	1,016	776
Investments	208	277
Recurring | Level 2 | Other debt securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	14,838	11,295
Investments	5,663	4,876
Recurring | Level 2 | Non-marketable equity securities measured using the measurement alternative
Assets, Fair Value Disclosure [Abstract]
Investments	69	50
Recurring | Level 2 | Trading non-derivative assets
Assets, Fair Value Disclosure [Abstract]
Trading account assets	115,747	106,023
Recurring | Level 2 | Trading account derivatives
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	381,430	464,562
Recurring | Level 2 | Trading account derivatives | Interest rate contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	183,139	238,026
Recurring | Level 2 | Trading account derivatives | Foreign exchange contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	118,018	155,994
Recurring | Level 2 | Trading account derivatives | Equity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	46,973	48,362
Recurring | Level 2 | Trading account derivatives | Commodity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	24,306	13,546
Recurring | Level 2 | Trading account derivatives | Credit derivatives
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	8,994	8,634
Recurring | Level 2 | Non-trading derivatives and other financial assets measured on a recurring basis
Assets, Fair Value Disclosure [Abstract]
Other assets, gross	7,571	8,383
Recurring | Level 3
Assets, Fair Value Disclosure [Abstract]
Federal funds sold and securities borrowed and purchased under agreements to resell	211	320
Investments	2,246	1,542
Loans held at fair value	429	1,985
Mortgage servicing rights	419	336
Assets before netting	$ 12,967	$ 16,135
Total as a percentage of gross assets	1.00%	1.10%
Liabilities, Fair Value Disclosure [Abstract]
Interest-bearing deposits	$ 154	$ 206
Federal funds purchased and securities loaned and sold under agreements to repurchase, Gross	488	631
Securities sold, not yet purchased	168	214
Trading liabilities	169	240
Short-term borrowings	41	219
Long-term debt	25,068	25,210
Total liabilities, Gross	$ 33,414	$ 36,019
Total as a percentage of gross liabilities	4.50%	4.50%
Recurring | Level 3 | Trading derivatives
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	$ 7,490	$ 9,512
Recurring | Level 3 | Trading derivatives | Interest rate contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	1,013	1,779
Recurring | Level 3 | Trading derivatives | Foreign exchange contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	737	622
Recurring | Level 3 | Trading derivatives | Equity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	4,092	5,304
Recurring | Level 3 | Trading derivatives | Commodity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	749	700
Recurring | Level 3 | Trading derivatives | Credit derivatives
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	899	1,107
Recurring | Level 3 | Non-trading derivatives and other financial liabilities measured on a recurring basis
Liabilities, Fair Value Disclosure [Abstract]
Other liabilities, gross	4	1
Recurring | Level 3 | Trading account liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	1	26
Recurring | Level 3 | Mortgage-backed securities - U.S. government-sponsored agency guaranteed
Assets, Fair Value Disclosure [Abstract]
Trading account assets	376	27
Investments	52	30
Recurring | Level 3 | Residential mortgages
Assets, Fair Value Disclosure [Abstract]
Trading account assets	95	340
Investments	0	0
Recurring | Level 3 | Commercial and other
Assets, Fair Value Disclosure [Abstract]
Trading account assets	87	136
Investments	0	0
Recurring | Level 3 | Mortgage-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	558	503
Investments	52	30
Recurring | Level 3 | U.S. Treasury and federal agency securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	0	0
Investments	0	0
Recurring | Level 3 | State and municipal
Assets, Fair Value Disclosure [Abstract]
Trading account assets	70	94
Investments	748	834
Recurring | Level 3 | Foreign government
Assets, Fair Value Disclosure [Abstract]
Trading account assets	141	51
Investments	957	268
Recurring | Level 3 | Corporate
Assets, Fair Value Disclosure [Abstract]
Trading account assets	823	375
Investments	104	60
Recurring | Level 3 | Equity securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	147	73
Investments	0	0
Recurring | Level 3 | Asset-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	692	1,606
Investments	3	1
Recurring | Level 3 | Other debt securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	555	945
Investments	0	0
Recurring | Level 3 | Non-marketable equity securities measured using the measurement alternative
Assets, Fair Value Disclosure [Abstract]
Investments	382	349
Recurring | Level 3 | Trading non-derivative assets
Assets, Fair Value Disclosure [Abstract]
Trading account assets	2,986	3,647
Recurring | Level 3 | Trading account derivatives
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	6,621	8,305
Recurring | Level 3 | Trading account derivatives | Interest rate contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	2,777	3,393
Recurring | Level 3 | Trading account derivatives | Foreign exchange contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	553	674
Recurring | Level 3 | Trading account derivatives | Equity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	1,542	2,091
Recurring | Level 3 | Trading account derivatives | Commodity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	891	992
Recurring | Level 3 | Trading account derivatives | Credit derivatives
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	858	1,155
Recurring | Level 3 | Non-trading derivatives and other financial assets measured on a recurring basis
Assets, Fair Value Disclosure [Abstract]
Other assets, gross	$ 55	$ 0