FAIR VALUE MEASUREMENT - Items Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Assets, Fair Value Disclosure [Abstract]
Federal funds sold and securities borrowed and purchased under agreements to resell, Netting	$ (121,511)	$ (157,370)
Trading account assets	342,914	375,079
Netting of cash collateral received	(48,391)	(63,915)
Investments	500,849	447,359
Loans held at fair value	7,159	6,854
Liabilities, Fair Value Disclosure [Abstract]
Federal funds purchased and securities loaned and sold under agreements to repurchase, Netting	(121,511)	(157,370)
Netting of cash collateral paid	(49,460)	(45,628)
Fair value
Liabilities, Fair Value Disclosure [Abstract]
Investments measured at net asset value excluded from Level 3	145	152
Recurring
Assets, Fair Value Disclosure [Abstract]
Federal funds sold and securities borrowed and purchased under agreements to resell	328,739	335,393
Federal funds sold and securities borrowed and purchased under agreements to resell, Netting	(116,539)	(150,189)
Federal funds sold and securities borrowed and purchased under agreements to resell	212,200	185,204
Investments	296,346	335,998
Loans held at fair value	7,159	6,854
Mortgage servicing rights	409	336
Assets before netting	1,349,379	1,497,067
Netting, Assets, total of netting agreements and cash collateral received	(478,456)	(578,983)
Total assets	870,923	918,084
Liabilities, Fair Value Disclosure [Abstract]
Interest-bearing deposits	2,713	1,958
Federal funds purchased and securities loaned and sold under agreements to repurchase, Gross	184,798	157,275
Federal funds purchased and securities loaned and sold under agreements to repurchase, Netting	(109,536)	(97,069)
Federal funds purchased and securities loaned and sold under agreements to repurchase	75,262	60,206
Securities sold, not yet purchased	116,444	100,044
Trading liabilities	116,451	100,070
Short-term borrowings	8,814	4,683
Long-term debt	78,178	67,063
Total liabilities, Gross	820,198	816,348
Total liabilities, Netting	(472,522)	(507,576)
Total liabilities	347,676	308,772
Recurring | Trading derivatives
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	407,329	470,268
Cash collateral received	18,492	8,196
Total trading derivatives and cash collateral, liability	425,821	478,464
Netting agreements	(313,526)	(364,879)
Netting of cash collateral paid	(49,460)	(45,628)
Netting, Liabilities, total of netting agreements and cash collateral received	(362,986)	(410,507)
Total derivative liabilities	62,835	67,957
Cash collateral received, gross	66,883	72,111
Recurring | Trading derivatives | Interest rate contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	156,899	222,411
Recurring | Trading derivatives | Foreign exchange contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	129,990	156,066
Recurring | Trading derivatives | Equity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	62,573	63,569
Recurring | Trading derivatives | Commodity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	47,218	18,093
Recurring | Trading derivatives | Credit derivatives
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	10,649	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	3,423	6,835
Total other assets and cash collateral, gross	3,423	6,835
Recurring | Trading account liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	7	26
Recurring | Mortgage-backed securities - U.S. government-sponsored agency guaranteed
Assets, Fair Value Disclosure [Abstract]
Trading account assets	30,009	42,930
Investments	36,669	43,918
Recurring | Residential mortgages
Assets, Fair Value Disclosure [Abstract]
Trading account assets	476	731
Investments	363	571
Recurring | Commercial and other
Assets, Fair Value Disclosure [Abstract]
Trading account assets	776	1,029
Investments	34	50
Recurring | Mortgage-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	31,261	44,690
Investments	37,066	44,539
Recurring | U.S. Treasury and federal agency securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	47,310	66,798
Investments	123,174	146,204
Recurring | State and municipal
Assets, Fair Value Disclosure [Abstract]
Trading account assets	1,641	1,318
Investments	2,684	3,719
Recurring | Foreign government
Assets, Fair Value Disclosure [Abstract]
Trading account assets	82,472	83,389
Investments	120,194	124,968
Recurring | Corporate
Assets, Fair Value Disclosure [Abstract]
Trading account assets	23,288	20,822
Investments	6,926	10,500
Recurring | Equity securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	60,268	66,479
Investments	379	515
Recurring | Asset-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	1,468	2,382
Investments	264	278
Recurring | Other debt securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	17,193	12,240
Investments	5,265	4,876
Recurring | Non-marketable equity securities measured using the measurement alternative
Assets, Fair Value Disclosure [Abstract]
Investments	394	399
Recurring | Trading non-derivative assets
Assets, Fair Value Disclosure [Abstract]
Trading account assets	264,901	298,118
Recurring | Trading account derivatives
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	416,714	472,977
Cash collateral paid	23,216	32,778
Trading derivative, asset, gross net cash collateral paid	439,930	505,755
Netting agreements	(313,526)	(364,879)
Netting of cash collateral received	(48,391)	(63,915)
Total trading derivatives, netting	(361,917)	(428,794)
Trading derivatives	78,013	76,961
Liabilities, Fair Value Disclosure [Abstract]
Cash collateral paid, gross	72,676	78,406
Recurring | Trading account derivatives | Interest rate contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	173,396	241,461
Recurring | Trading account derivatives | Foreign exchange contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	132,260	156,670
Recurring | Trading account derivatives | Equity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	53,681	50,519
Recurring | Trading account derivatives | Commodity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	47,135	14,538
Recurring | Trading account derivatives | Credit derivatives
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	10,242	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	11,895	14,613
Other assets	11,895	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	192,138	229,701
Loans held at fair value	0	0
Mortgage servicing rights	0	0
Assets before netting	$ 345,782	$ 424,489
Total as a percentage of gross assets	26.10%	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	98,685	85,353
Trading liabilities	98,685	85,353
Short-term borrowings	0	0
Long-term debt	0	0
Total liabilities, Gross	$ 102,004	$ 92,196
Total as a percentage of gross liabilities	12.70%	11.40%
Recurring | Level 1 | Trading derivatives
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	$ 74	$ 81
Recurring | Level 1 | Trading derivatives | Interest rate contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	45	25
Recurring | Level 1 | Trading derivatives | Foreign exchange contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	3	3
Recurring | Level 1 | Trading derivatives | Equity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	26	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	3,245	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	42,689	64,529
Investments	123,004	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	54,922	68,195
Investments	65,314	77,056
Recurring | Level 1 | Corporate
Assets, Fair Value Disclosure [Abstract]
Trading account assets	1,914	1,607
Investments	3,577	6,326
Recurring | Level 1 | Equity securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	49,713	54,117
Investments	243	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	0	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	149,238	188,448
Recurring | Level 1 | Trading account derivatives
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	59	110
Recurring | Level 1 | Trading account derivatives | Interest rate contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	59	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	4,347	6,230
Recurring | Level 2
Assets, Fair Value Disclosure [Abstract]
Federal funds sold and securities borrowed and purchased under agreements to resell	328,482	335,073
Investments	102,055	104,755
Loans held at fair value	6,437	4,869
Mortgage servicing rights	0	0
Assets before netting	$ 966,235	$ 1,023,665
Total as a percentage of gross assets	72.90%	69.90%
Liabilities, Fair Value Disclosure [Abstract]
Interest-bearing deposits	$ 2,531	$ 1,752
Federal funds purchased and securities loaned and sold under agreements to repurchase, Gross	184,142	156,644
Securities sold, not yet purchased	17,671	14,477
Trading liabilities	17,678	14,477
Short-term borrowings	8,811	4,464
Long-term debt	52,136	41,853
Total liabilities, Gross	$ 665,031	$ 679,937
Total as a percentage of gross liabilities	83.00%	84.10%
Recurring | Level 2 | Trading derivatives
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	$ 399,556	$ 460,675
Recurring | Level 2 | Trading derivatives | Interest rate contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	155,183	220,607
Recurring | Level 2 | Trading derivatives | Foreign exchange contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	129,263	155,441
Recurring | Level 2 | Trading derivatives | Equity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	58,787	58,212
Recurring | Level 2 | Trading derivatives | Commodity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	46,434	17,393
Recurring | Level 2 | Trading derivatives | Credit derivatives
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	9,889	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	177	72
Recurring | Level 2 | Trading account liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	7	0
Recurring | Level 2 | Mortgage-backed securities - U.S. government-sponsored agency guaranteed
Assets, Fair Value Disclosure [Abstract]
Trading account assets	29,577	42,903
Investments	36,617	43,888
Recurring | Level 2 | Residential mortgages
Assets, Fair Value Disclosure [Abstract]
Trading account assets	415	391
Investments	351	571
Recurring | Level 2 | Commercial and other
Assets, Fair Value Disclosure [Abstract]
Trading account assets	656	893
Investments	34	50
Recurring | Level 2 | Mortgage-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	30,648	44,187
Investments	37,002	44,509
Recurring | Level 2 | U.S. Treasury and federal agency securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	4,621	2,269
Investments	170	172
Recurring | Level 2 | State and municipal
Assets, Fair Value Disclosure [Abstract]
Trading account assets	1,570	1,224
Investments	1,949	2,885
Recurring | Level 2 | Foreign government
Assets, Fair Value Disclosure [Abstract]
Trading account assets	27,517	15,143
Investments	54,068	47,644
Recurring | Level 2 | Corporate
Assets, Fair Value Disclosure [Abstract]
Trading account assets	20,833	18,840
Investments	3,157	4,114
Recurring | Level 2 | Equity securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	10,350	12,289
Investments	136	228
Recurring | Level 2 | Asset-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	804	776
Investments	261	277
Recurring | Level 2 | Other debt securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	16,278	11,295
Investments	5,265	4,876
Recurring | Level 2 | Non-marketable equity securities measured using the measurement alternative
Assets, Fair Value Disclosure [Abstract]
Investments	47	50
Recurring | Level 2 | Trading non-derivative assets
Assets, Fair Value Disclosure [Abstract]
Trading account assets	112,621	106,023
Recurring | Level 2 | Trading account derivatives
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	409,178	464,562
Recurring | Level 2 | Trading account derivatives | Interest rate contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	170,175	238,026
Recurring | Level 2 | Trading account derivatives | Foreign exchange contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	131,744	155,994
Recurring | Level 2 | Trading account derivatives | Equity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	51,894	48,362
Recurring | Level 2 | Trading account derivatives | Commodity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	45,933	13,546
Recurring | Level 2 | Trading account derivatives | Credit derivatives
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	9,432	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	7,462	8,383
Recurring | Level 3
Assets, Fair Value Disclosure [Abstract]
Federal funds sold and securities borrowed and purchased under agreements to resell	257	320
Investments	2,153	1,542
Loans held at fair value	722	1,985
Mortgage servicing rights	409	336
Assets before netting	$ 14,146	$ 16,135
Total as a percentage of gross assets	1.10%	1.10%
Liabilities, Fair Value Disclosure [Abstract]
Interest-bearing deposits	$ 182	$ 206
Federal funds purchased and securities loaned and sold under agreements to repurchase, Gross	656	631
Securities sold, not yet purchased	88	214
Trading liabilities	88	240
Short-term borrowings	3	219
Long-term debt	26,042	25,210
Total liabilities, Gross	$ 34,671	$ 36,019
Total as a percentage of gross liabilities	4.30%	4.50%
Recurring | Level 3 | Trading derivatives
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	$ 7,699	$ 9,512
Recurring | Level 3 | Trading derivatives | Interest rate contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	1,671	1,779
Recurring | Level 3 | Trading derivatives | Foreign exchange contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	724	622
Recurring | Level 3 | Trading derivatives | Equity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	3,760	5,304
Recurring | Level 3 | Trading derivatives | Commodity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	784	700
Recurring | Level 3 | Trading derivatives | Credit derivatives
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	760	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	1	1
Recurring | Level 3 | Trading account liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	0	26
Recurring | Level 3 | Mortgage-backed securities - U.S. government-sponsored agency guaranteed
Assets, Fair Value Disclosure [Abstract]
Trading account assets	432	27
Investments	52	30
Recurring | Level 3 | Residential mortgages
Assets, Fair Value Disclosure [Abstract]
Trading account assets	61	340
Investments	12	0
Recurring | Level 3 | Commercial and other
Assets, Fair Value Disclosure [Abstract]
Trading account assets	120	136
Investments	0	0
Recurring | Level 3 | Mortgage-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	613	503
Investments	64	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	71	94
Investments	735	834
Recurring | Level 3 | Foreign government
Assets, Fair Value Disclosure [Abstract]
Trading account assets	33	51
Investments	812	268
Recurring | Level 3 | Corporate
Assets, Fair Value Disclosure [Abstract]
Trading account assets	541	375
Investments	192	60
Recurring | Level 3 | Equity securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	205	73
Investments	0	0
Recurring | Level 3 | Asset-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	664	1,606
Investments	3	1
Recurring | Level 3 | Other debt securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	915	945
Investments	0	0
Recurring | Level 3 | Non-marketable equity securities measured using the measurement alternative
Assets, Fair Value Disclosure [Abstract]
Investments	347	349
Recurring | Level 3 | Trading non-derivative assets
Assets, Fair Value Disclosure [Abstract]
Trading account assets	3,042	3,647
Recurring | Level 3 | Trading account derivatives
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	7,477	8,305
Recurring | Level 3 | Trading account derivatives | Interest rate contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	3,162	3,393
Recurring | Level 3 | Trading account derivatives | Foreign exchange contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	516	674
Recurring | Level 3 | Trading account derivatives | Equity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	1,787	2,091
Recurring | Level 3 | Trading account derivatives | Commodity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	1,202	992
Recurring | Level 3 | Trading account derivatives | Credit derivatives
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	810	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	$ 86	$ 0