FAIR VALUE MEASUREMENT - Items Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Assets, Fair Value Disclosure [Abstract]
Federal funds sold and securities borrowed and purchased under agreements to resell, Netting	$ (115,215)	$ (111,400)
Trading account assets	348,209	276,140
Netting of cash collateral received	(54,089)	(44,353)
Investments	447,524	368,563
Loans at fair value	5,524	4,085
Liabilities, Fair Value Disclosure [Abstract]
Federal funds purchased and securities loaned and sold under agreements to repurchase, Netting	(115,215)	(111,400)
Netting of cash collateral paid	(53,126)	(38,919)
Fair value
Liabilities, Fair Value Disclosure [Abstract]
Investments measured at net asset value excluded from Level 3	142	170
Recurring
Assets, Fair Value Disclosure [Abstract]
Federal funds sold and securities borrowed and purchased under agreements to resell	285,996	254,556
Federal funds sold and securities borrowed and purchased under agreements to resell, Netting	(110,370)	(101,363)
Federal funds sold and securities borrowed and purchased under agreements to resell	175,626	153,193
Investments	344,746	281,257
Loans at fair value	5,524	4,085
Mortgage servicing rights	334	495
Assets before netting	1,381,583	1,148,686
Netting, Assets, total of netting agreements and cash collateral received	(495,452)	(420,686)
Total assets	886,131	728,000
Liabilities, Fair Value Disclosure [Abstract]
Interest-bearing deposits	2,544	2,319
Federal funds purchased and securities loaned and sold under agreements to repurchase, Gross	134,711	112,324
Federal funds purchased and securities loaned and sold under agreements to repurchase, Netting	(73,833)	(71,673)
Federal funds purchased and securities loaned and sold under agreements to repurchase	60,878	40,651
Securities sold, not yet purchased	89,913	72,442
Trading liabilities	89,956	72,466
Short-term borrowings	5,981	4,946
Long-term debt	63,056	55,783
Total liabilities, Gross	742,468	615,498
Total liabilities, Netting	(457,952)	(385,562)
Total liabilities	284,516	229,936
Recurring | Trading account liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	424,099	346,926
Cash collateral received	17,054	14,391
Total trading derivatives and cash collateral, liability	441,153	361,317
Netting agreements	(330,993)	(274,970)
Netting of cash collateral paid	(53,126)	(38,919)
Netting, Liabilities, total of netting agreements and cash collateral received	(384,119)	(313,889)
Total derivative liabilities	57,034	47,428
Cash collateral received, gross	71,143	58,744
Recurring | Trading account liabilities | Interest rate contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	229,181	177,655
Recurring | Trading account liabilities | Foreign exchange contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	111,800	110,732
Recurring | Trading account liabilities | Equity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	52,599	30,486
Recurring | Trading account liabilities | Commodity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	19,025	17,315
Recurring | Trading account liabilities | Credit derivatives
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	11,494	10,738
Recurring | Non-trading derivatives and other financial liabilities
Liabilities, Fair Value Disclosure [Abstract]
Netting of cash collateral paid		0
Netting, Liabilities, total of netting agreements and cash collateral received	0
Non-trading derivatives and other financial liabilities measured on a recurring basis, gross	5,067	6,343
Total other assets and cash collateral, gross	5,067	6,343
Recurring | Trading account liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	43	24
Recurring | Mortgage-backed securities - U.S. government-sponsored agency guaranteed
Assets, Fair Value Disclosure [Abstract]
Trading account assets	37,410	27,671
Investments	42,718	35,230
Recurring | Residential mortgages
Assets, Fair Value Disclosure [Abstract]
Trading account assets	638	696
Investments	581	793
Recurring | Commercial and other
Assets, Fair Value Disclosure [Abstract]
Trading account assets	1,193	1,693
Investments	61	74
Recurring | Mortgage-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	39,241	30,060
Investments	43,360	36,097
Recurring | U.S. Treasury and federal agency securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	67,100	29,895
Investments	160,117	111,418
Recurring | State and municipal
Assets, Fair Value Disclosure [Abstract]
Trading account assets	1,336	2,637
Investments	4,116	4,978
Recurring | Foreign government
Assets, Fair Value Disclosure [Abstract]
Trading account assets	87,184	71,326
Investments	120,563	111,249
Recurring | Corporate
Assets, Fair Value Disclosure [Abstract]
Trading account assets	20,341	18,891
Investments	10,730	11,271
Recurring | Equity securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	50,903	51,641
Investments	693	458
Recurring | Asset-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	2,615	2,716
Investments	275	522
Recurring | Other debt securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	12,221	12,041
Investments	4,529	4,730
Recurring | Non-marketable equity securities measured using the measurement alternative
Assets, Fair Value Disclosure [Abstract]
Investments	363	534
Recurring | Trading securities (excluding trading account derivatives)
Assets, Fair Value Disclosure [Abstract]
Trading account assets	280,941	219,207
Recurring | Trading account assets
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	429,191	358,330
Cash collateral paid	23,159	17,926
Trading derivative, asset, gross net cash collateral paid	452,350	376,256
Netting agreements	(330,993)	(274,970)
Netting of cash collateral received	(54,089)	(44,353)
Total trading derivatives, netting	(385,082)	(319,323)
Trading derivatives	67,268	56,933
Liabilities, Fair Value Disclosure [Abstract]
Cash collateral paid, gross	76,285	56,845
Recurring | Trading account assets | Interest rate contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	251,167	197,668
Recurring | Trading account assets | Foreign exchange contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	110,351	107,570
Recurring | Trading account assets | Equity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	41,792	28,471
Recurring | Trading account assets | Commodity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	14,997	14,212
Recurring | Trading account assets | Credit derivatives
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	10,884	10,409
Recurring | Non-trading derivatives and other financial assets
Assets, Fair Value Disclosure [Abstract]
Total trading derivatives, netting	0	0
Total other assets and cash collateral, gross	11,692	12,830
Other assets	11,692	12,830
Recurring | Level 1
Assets, Fair Value Disclosure [Abstract]
Federal funds sold and securities borrowed and purchased under agreements to resell	0	0
Investments	243,007	181,297
Loans at fair value	0	0
Mortgage servicing rights	0	0
Assets before netting	$ 425,960	$ 307,192
Total as a percentage of gross assets	31.40%	27.20%
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	78,119	60,429
Trading liabilities	78,119	60,429
Short-term borrowings	0	0
Long-term debt	0	0
Total liabilities, Gross	$ 83,081	$ 66,861
Total as a percentage of gross liabilities	11.50%	11.10%
Recurring | Level 1 | Trading account liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	$ 74	$ 152
Recurring | Level 1 | Trading account liabilities | Interest rate contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	35	8
Recurring | Level 1 | Trading account liabilities | Foreign exchange contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	1	0
Recurring | Level 1 | Trading account liabilities | Equity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	38	144
Recurring | Level 1 | Trading account liabilities | Commodity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	0	0
Recurring | Level 1 | Trading account 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	4,888	6,280
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	7	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	7	0
Investments	0	0
Recurring | Level 1 | U.S. Treasury and federal agency securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	64,967	26,159
Investments	159,944	106,103
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	70,610	50,948
Investments	76,172	69,957
Recurring | Level 1 | Corporate
Assets, Fair Value Disclosure [Abstract]
Trading account assets	2,100	1,332
Investments	6,545	5,150
Recurring | Level 1 | Equity securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	40,357	41,663
Investments	346	87
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	456	74
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 securities (excluding trading account derivatives)
Assets, Fair Value Disclosure [Abstract]
Trading account assets	178,497	120,176
Recurring | Level 1 | Trading account assets
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	107	91
Recurring | Level 1 | Trading account assets | Interest rate contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	44	7
Recurring | Level 1 | Trading account assets | Foreign exchange contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	1	1
Recurring | Level 1 | Trading account assets | Equity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	62	83
Recurring | Level 1 | Trading account assets | Commodity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	0	0
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	4,349	5,628
Recurring | Level 2
Assets, Fair Value Disclosure [Abstract]
Federal funds sold and securities borrowed and purchased under agreements to resell	285,666	254,253
Investments	100,287	98,701
Loans at fair value	3,843	3,683
Mortgage servicing rights	0	0
Assets before netting	$ 918,017	$ 815,535
Total as a percentage of gross assets	67.60%	72.10%
Liabilities, Fair Value Disclosure [Abstract]
Interest-bearing deposits	$ 2,331	$ 2,104
Federal funds purchased and securities loaned and sold under agreements to repurchase, Gross	134,052	111,567
Securities sold, not yet purchased	11,653	11,965
Trading liabilities	11,653	11,989
Short-term borrowings	5,780	4,933
Long-term debt	39,940	38,614
Total liabilities, Gross	$ 610,128	$ 511,211
Total as a percentage of gross liabilities	84.10%	85.00%
Recurring | Level 2 | Trading account liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	$ 416,195	$ 341,941
Recurring | Level 2 | Trading account liabilities | Interest rate contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	227,527	176,480
Recurring | Level 2 | Trading account liabilities | Foreign exchange contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	111,203	110,180
Recurring | Level 2 | Trading account liabilities | Equity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	49,371	28,506
Recurring | Level 2 | Trading account liabilities | Commodity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	17,816	16,542
Recurring | Level 2 | Trading account liabilities | Credit derivatives
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	10,280	10,233
Recurring | Level 2 | Non-trading derivatives and other financial liabilities
Liabilities, Fair Value Disclosure [Abstract]
Other liabilities, gross	177	63
Recurring | Level 2 | Trading account liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	0	24
Recurring | Level 2 | Mortgage-backed securities - U.S. government-sponsored agency guaranteed
Assets, Fair Value Disclosure [Abstract]
Trading account assets	37,144	27,661
Investments	42,688	35,198
Recurring | Level 2 | Residential mortgages
Assets, Fair Value Disclosure [Abstract]
Trading account assets	362	573
Investments	581	793
Recurring | Level 2 | Commercial and other
Assets, Fair Value Disclosure [Abstract]
Trading account assets	1,012	1,632
Investments	61	74
Recurring | Level 2 | Mortgage-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	38,518	29,866
Investments	43,330	36,065
Recurring | Level 2 | U.S. Treasury and federal agency securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	2,133	3,736
Investments	173	5,315
Recurring | Level 2 | State and municipal
Assets, Fair Value Disclosure [Abstract]
Trading account assets	1,260	2,573
Investments	3,291	4,355
Recurring | Level 2 | Foreign government
Assets, Fair Value Disclosure [Abstract]
Trading account assets	16,533	20,326
Investments	44,202	41,196
Recurring | Level 2 | Corporate
Assets, Fair Value Disclosure [Abstract]
Trading account assets	17,967	17,246
Investments	4,129	6,076
Recurring | Level 2 | Equity securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	10,481	9,878
Investments	347	371
Recurring | Level 2 | Asset-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	787	1,539
Investments	273	500
Recurring | Level 2 | Other debt securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	10,971	11,412
Investments	4,529	4,730
Recurring | Level 2 | Non-marketable equity securities measured using the measurement alternative
Assets, Fair Value Disclosure [Abstract]
Investments	13	93
Recurring | Level 2 | Trading securities (excluding trading account derivatives)
Assets, Fair Value Disclosure [Abstract]
Trading account assets	98,650	96,576
Recurring | Level 2 | Trading account assets
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	422,228	355,121
Recurring | Level 2 | Trading account assets | Interest rate contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	247,968	196,493
Recurring | Level 2 | Trading account assets | Foreign exchange contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	109,729	107,022
Recurring | Level 2 | Trading account assets | Equity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	41,199	28,148
Recurring | Level 2 | Trading account assets | Commodity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	13,813	13,498
Recurring | Level 2 | Trading account assets | Credit derivatives
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	9,519	9,960
Recurring | Level 2 | Non-trading derivatives and other financial assets
Assets, Fair Value Disclosure [Abstract]
Other assets, gross	7,343	7,201
Recurring | Level 3
Assets, Fair Value Disclosure [Abstract]
Federal funds sold and securities borrowed and purchased under agreements to resell	330	303
Investments	1,452	1,259
Loans at fair value	1,681	402
Mortgage servicing rights	334	495
Assets before netting	$ 14,447	$ 8,033
Total as a percentage of gross assets	1.10%	0.70%
Liabilities, Fair Value Disclosure [Abstract]
Interest-bearing deposits	$ 213	$ 215
Federal funds purchased and securities loaned and sold under agreements to repurchase, Gross	659	757
Securities sold, not yet purchased	141	48
Trading liabilities	184	48
Short-term borrowings	201	13
Long-term debt	23,116	17,169
Total liabilities, Gross	$ 32,203	$ 23,035
Total as a percentage of gross liabilities	4.40%	3.80%
Recurring | Level 3 | Trading account liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	$ 7,828	$ 4,833
Recurring | Level 3 | Trading account liabilities | Interest rate contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	1,619	1,167
Recurring | Level 3 | Trading account liabilities | Foreign exchange contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	596	552
Recurring | Level 3 | Trading account liabilities | Equity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	3,190	1,836
Recurring | Level 3 | Trading account liabilities | Commodity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	1,209	773
Recurring | Level 3 | Trading account liabilities | Credit derivatives
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	1,214	505
Recurring | Level 3 | Non-trading derivatives and other financial liabilities
Liabilities, Fair Value Disclosure [Abstract]
Other liabilities, gross	2	0
Recurring | Level 3 | Trading account liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	43	0
Recurring | Level 3 | Mortgage-backed securities - U.S. government-sponsored agency guaranteed
Assets, Fair Value Disclosure [Abstract]
Trading account assets	266	10
Investments	30	32
Recurring | Level 3 | Residential mortgages
Assets, Fair Value Disclosure [Abstract]
Trading account assets	269	123
Investments	0	0
Recurring | Level 3 | Commercial and other
Assets, Fair Value Disclosure [Abstract]
Trading account assets	181	61
Investments	0	0
Recurring | Level 3 | Mortgage-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	716	194
Investments	30	32
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	76	64
Investments	825	623
Recurring | Level 3 | Foreign government
Assets, Fair Value Disclosure [Abstract]
Trading account assets	41	52
Investments	189	96
Recurring | Level 3 | Corporate
Assets, Fair Value Disclosure [Abstract]
Trading account assets	274	313
Investments	56	45
Recurring | Level 3 | Equity securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	65	100
Investments	0	0
Recurring | Level 3 | Asset-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	1,828	1,177
Investments	2	22
Recurring | Level 3 | Other debt securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	794	555
Investments	0	0
Recurring | Level 3 | Non-marketable equity securities measured using the measurement alternative
Assets, Fair Value Disclosure [Abstract]
Investments	350	441
Recurring | Level 3 | Trading securities (excluding trading account derivatives)
Assets, Fair Value Disclosure [Abstract]
Trading account assets	3,794	2,455
Recurring | Level 3 | Trading account assets
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	6,856	3,118
Recurring | Level 3 | Trading account assets | Interest rate contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	3,155	1,168
Recurring | Level 3 | Trading account assets | Foreign exchange contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	621	547
Recurring | Level 3 | Trading account assets | Equity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	531	240
Recurring | Level 3 | Trading account assets | Commodity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	1,184	714
Recurring | Level 3 | Trading account assets | Credit derivatives
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	1,365	449
Recurring | Level 3 | Non-trading derivatives and other financial assets
Assets, Fair Value Disclosure [Abstract]
Other assets, gross	$ 0	$ 1