FAIR VALUE MEASUREMENT - Items Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Assets, Fair Value Disclosure [Abstract]
Federal funds sold and securities borrowed and purchased under agreements to resell, Netting	$ (128,302)	$ (87,424)
Trading account assets	306,831	256,117
Netting of cash collateral received	(47,136)	(38,608)
Investments	349,702	358,607
Loans at fair value	3,824	3,223
Liabilities, Fair Value Disclosure [Abstract]
Federal funds purchased and securities loaned and sold under agreements to repurchase, Netting	(128,302)	(87,424)
Netting of cash collateral paid	(37,933)	(29,911)
Fair value
Liabilities, Fair Value Disclosure [Abstract]
Investments measured at net asset value excluded from Level 3	196	207
Fair value | Accounting Standards Update 2015-07
Liabilities, Fair Value Disclosure [Abstract]
Investments measured at net asset value excluded from Level 3	200	200
Recurring
Assets, Fair Value Disclosure [Abstract]
Federal funds sold and securities borrowed and purchased under agreements to resell	263,621	214,685
Federal funds sold and securities borrowed and purchased under agreements to resell, Netting	(85,513)	(66,984)
Federal funds sold and securities borrowed and purchased under agreements to resell	178,108	147,701
Investments	274,512	288,940
Loans at fair value	3,824	3,223
Mortgage servicing rights	508	584
Assets before netting	1,230,452	1,134,034
Netting, Assets, total of netting agreements and cash collateral received	(444,072)	(416,681)
Total assets	786,380	717,353
Liabilities, Fair Value Disclosure [Abstract]
Interest-bearing deposits	2,639	1,475
Federal funds purchased and securities loaned and sold under agreements to repurchase, Gross	130,650	111,494
Federal funds purchased and securities loaned and sold under agreements to repurchase, Netting	(85,513)	(66,984)
Federal funds purchased and securities loaned and sold under agreements to repurchase	45,137	44,510
Securities sold, not yet purchased	86,484	90,822
Trading liabilities	86,521	92,369
Short-term borrowings	5,291	4,483
Long-term debt	49,488	38,229
Total liabilities, Gross	690,517	656,892
Total liabilities, Netting	(434,869)	(407,984)
Total liabilities	255,648	248,908
Recurring | Trading account liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	385,088	379,030
Cash collateral received	14,041	13,906
Total trading derivatives and cash collateral, liability	399,129	392,936
Netting agreements	(311,423)	(311,089)
Netting of cash collateral paid	(37,933)	(29,911)
Netting, Liabilities, total of netting agreements and cash collateral received	(349,356)	(341,000)
Total derivative liabilities	49,773	51,936
Cash collateral received, gross	61,177	52,514
Recurring | Trading account liabilities | Interest rate contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	190,761	154,827
Recurring | Trading account liabilities | Foreign exchange contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	132,830	159,355
Recurring | Trading account liabilities | Equity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	31,794	33,808
Recurring | Trading account liabilities | Commodity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	17,783	20,689
Recurring | Trading account liabilities | Credit derivatives
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	11,920	10,351
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	16,799	15,906
Total other assets and cash collateral, gross	16,799	15,906
Recurring | Trading account liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	37	1,547
Recurring | Mortgage-backed securities - U.S. agency-sponsored
Assets, Fair Value Disclosure [Abstract]
Trading account assets	27,223	24,246
Investments	37,860	43,020
Recurring | Residential mortgage
Assets, Fair Value Disclosure [Abstract]
Trading account assets	671	977
Investments	910	1,313
Recurring | Commercial and other
Assets, Fair Value Disclosure [Abstract]
Trading account assets	1,535	1,400
Investments	115	172
Recurring | Mortgage-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	29,429	26,623
Investments	38,885	44,505
Recurring | U.S. Treasury and federal agency securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	43,313	31,242
Investments	109,509	117,222
Recurring | State and municipal securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	2,848	3,982
Investments	6,170	9,206
Recurring | Foreign government
Assets, Fair Value Disclosure [Abstract]
Trading account assets	84,065	64,519
Investments	101,740	100,691
Recurring | Corporate
Assets, Fair Value Disclosure [Abstract]
Trading account assets	17,551	15,896
Investments	12,323	11,599
Recurring | Equity securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	60,865	43,803
Investments	533	220
Recurring | Asset-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	3,146	2,913
Investments	618	843
Recurring | Other debt securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	11,506	13,019
Investments	4,190	3,972
Recurring | Non-marketable equity securities measured using the measurement alternative
Assets, Fair Value Disclosure [Abstract]
Investments	544	682
Recurring | Trading securities (excluding trading account derivatives)
Assets, Fair Value Disclosure [Abstract]
Trading account assets	252,723	201,997
Recurring | Trading account assets
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	398,533	392,299
Cash collateral paid	14,134	11,518
Trading derivative, asset, gross net cash collateral paid	412,667	403,817
Netting agreements	(311,423)	(311,089)
Netting of cash collateral received	(47,136)	(38,608)
Total trading derivatives, netting	(358,559)	(349,697)
Trading derivatives	54,108	54,120
Liabilities, Fair Value Disclosure [Abstract]
Cash collateral paid, gross	52,067	41,429
Recurring | Trading account assets | Interest rate contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	213,644	171,632
Recurring | Trading account assets | Foreign exchange contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	131,974	162,454
Recurring | Trading account assets | Equity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	27,188	29,893
Recurring | Trading account assets | Commodity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	14,740	17,555
Recurring | Trading account assets | Credit derivatives
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	10,987	10,765
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	22,597	20,788
Other assets	22,597	20,788
Recurring | Level 1
Assets, Fair Value Disclosure [Abstract]
Federal funds sold and securities borrowed and purchased under agreements to resell	0	0
Investments	171,062	170,445
Loans at fair value	0	0
Mortgage servicing rights	0	0
Assets before netting	$ 340,285	$ 294,151
Total as a percentage of gross assets	28.00%	26.20%
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	73,084	78,872
Trading liabilities	73,084	78,872
Short-term borrowings	0	0
Long-term debt	0	0
Total liabilities, Gross	$ 90,260	$ 95,133
Total as a percentage of gross liabilities	13.30%	14.80%
Recurring | Level 1 | Trading account liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	$ 507	$ 422
Recurring | Level 1 | Trading account liabilities | Interest rate contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	211	71
Recurring | Level 1 | Trading account liabilities | Foreign exchange contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	0	0
Recurring | Level 1 | Trading account liabilities | Equity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	296	351
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	16,669	15,839
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 | Residential mortgage
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	38,082	26,439
Investments	101,889	107,577
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	59,912	43,309
Investments	64,001	58,252
Recurring | Level 1 | Corporate
Assets, Fair Value Disclosure [Abstract]
Trading account assets	2,288	1,026
Investments	5,115	4,410
Recurring | Level 1 | Equity securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	51,566	36,342
Investments	57	206
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	10	3
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	151,858	107,119
Recurring | Level 1 | Trading account assets
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	696	748
Recurring | Level 1 | Trading account assets | Interest rate contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	332	101
Recurring | Level 1 | Trading account assets | Foreign exchange contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	0	0
Recurring | Level 1 | Trading account assets | Equity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	364	647
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	16,669	15,839
Recurring | Level 2
Assets, Fair Value Disclosure [Abstract]
Federal funds sold and securities borrowed and purchased under agreements to resell	263,499	214,570
Investments	101,753	116,758
Loans at fair value	3,405	2,946
Mortgage servicing rights	0	0
Assets before netting	$ 865,780	$ 818,051
Total as a percentage of gross assets	71.20%	72.90%
Liabilities, Fair Value Disclosure [Abstract]
Interest-bearing deposits	$ 1,457	$ 980
Federal funds purchased and securities loaned and sold under agreements to repurchase, Gross	129,565	110,511
Securities sold, not yet purchased	13,372	11,364
Trading liabilities	13,409	12,911
Short-term borrowings	5,137	4,446
Long-term debt	34,550	25,659
Total liabilities, Gross	$ 563,450	$ 528,228
Total as a percentage of gross liabilities	83.30%	82.10%
Recurring | Level 2 | Trading account liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	$ 379,203	$ 373,654
Recurring | Level 2 | Trading account liabilities | Interest rate contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	188,797	152,931
Recurring | Level 2 | Trading account liabilities | Foreign exchange contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	132,354	159,003
Recurring | Level 2 | Trading account liabilities | Equity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	29,835	32,330
Recurring | Level 2 | Trading account liabilities | Commodity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	16,907	19,904
Recurring | Level 2 | Trading account liabilities | Credit derivatives
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	11,310	9,486
Recurring | Level 2 | Non-trading derivatives and other financial liabilities
Liabilities, Fair Value Disclosure [Abstract]
Other liabilities, gross	129	67
Recurring | Level 2 | Trading account liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	37	1,547
Recurring | Level 2 | Mortgage-backed securities - U.S. agency-sponsored
Assets, Fair Value Disclosure [Abstract]
Trading account assets	27,036	24,090
Investments	37,829	42,988
Recurring | Level 2 | Residential mortgage
Assets, Fair Value Disclosure [Abstract]
Trading account assets	540	709
Investments	910	1,313
Recurring | Level 2 | Commercial and other
Assets, Fair Value Disclosure [Abstract]
Trading account assets	1,482	1,323
Investments	115	172
Recurring | Level 2 | Mortgage-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	29,058	26,122
Investments	38,854	44,473
Recurring | Level 2 | U.S. Treasury and federal agency securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	5,231	4,802
Investments	7,620	9,645
Recurring | Level 2 | State and municipal securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	2,671	3,782
Investments	5,144	8,498
Recurring | Level 2 | Foreign government
Assets, Fair Value Disclosure [Abstract]
Trading account assets	24,133	21,179
Investments	37,662	42,371
Recurring | Level 2 | Corporate
Assets, Fair Value Disclosure [Abstract]
Trading account assets	14,809	14,510
Investments	7,152	7,033
Recurring | Level 2 | Equity securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	9,176	7,308
Investments	476	14
Recurring | Level 2 | Asset-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	1,735	1,429
Investments	559	656
Recurring | Level 2 | Other debt securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	10,756	12,198
Investments	4,190	3,972
Recurring | Level 2 | Non-marketable equity securities measured using the measurement alternative
Assets, Fair Value Disclosure [Abstract]
Investments	96	96
Recurring | Level 2 | Trading securities (excluding trading account derivatives)
Assets, Fair Value Disclosure [Abstract]
Trading account assets	97,569	91,330
Recurring | Level 2 | Trading account assets
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	393,626	387,498
Recurring | Level 2 | Trading account assets | Interest rate contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	211,668	169,860
Recurring | Level 2 | Trading account assets | Foreign exchange contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	131,595	162,108
Recurring | Level 2 | Trading account assets | Equity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	26,355	28,903
Recurring | Level 2 | Trading account assets | Commodity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	13,717	16,788
Recurring | Level 2 | Trading account assets | Credit derivatives
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	10,291	9,839
Recurring | Level 2 | Non-trading derivatives and other financial assets
Assets, Fair Value Disclosure [Abstract]
Other assets, gross	5,928	4,949
Recurring | Level 3
Assets, Fair Value Disclosure [Abstract]
Federal funds sold and securities borrowed and purchased under agreements to resell	122	115
Investments	1,697	1,737
Loans at fair value	419	277
Mortgage servicing rights	508	584
Assets before netting	$ 10,253	$ 10,314
Total as a percentage of gross assets	0.80%	0.90%
Liabilities, Fair Value Disclosure [Abstract]
Interest-bearing deposits	$ 1,182	$ 495
Federal funds purchased and securities loaned and sold under agreements to repurchase, Gross	1,085	983
Securities sold, not yet purchased	28	586
Trading liabilities	28	586
Short-term borrowings	154	37
Long-term debt	14,938	12,570
Total liabilities, Gross	$ 22,766	$ 19,625
Total as a percentage of gross liabilities	3.40%	3.10%
Recurring | Level 3 | Trading account liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	$ 5,378	$ 4,954
Recurring | Level 3 | Trading account liabilities | Interest rate contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	1,753	1,825
Recurring | Level 3 | Trading account liabilities | Foreign exchange contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	476	352
Recurring | Level 3 | Trading account liabilities | Equity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	1,663	1,127
Recurring | Level 3 | Trading account liabilities | Commodity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	876	785
Recurring | Level 3 | Trading account liabilities | Credit derivatives
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	610	865
Recurring | Level 3 | Non-trading derivatives and other financial liabilities
Liabilities, Fair Value Disclosure [Abstract]
Other liabilities, gross	1	0
Recurring | Level 3 | Trading account liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	0	0
Recurring | Level 3 | Mortgage-backed securities - U.S. agency-sponsored
Assets, Fair Value Disclosure [Abstract]
Trading account assets	187	156
Investments	31	32
Recurring | Level 3 | Residential mortgage
Assets, Fair Value Disclosure [Abstract]
Trading account assets	131	268
Investments	0	0
Recurring | Level 3 | Commercial and other
Assets, Fair Value Disclosure [Abstract]
Trading account assets	53	77
Investments	0	0
Recurring | Level 3 | Mortgage-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	371	501
Investments	31	32
Recurring | Level 3 | U.S. Treasury and federal agency securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	0	1
Investments	0	0
Recurring | Level 3 | State and municipal securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	177	200
Investments	1,026	708
Recurring | Level 3 | Foreign government
Assets, Fair Value Disclosure [Abstract]
Trading account assets	20	31
Investments	77	68
Recurring | Level 3 | Corporate
Assets, Fair Value Disclosure [Abstract]
Trading account assets	454	360
Investments	56	156
Recurring | Level 3 | Equity securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	123	153
Investments	0	0
Recurring | Level 3 | Asset-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	1,411	1,484
Investments	59	187
Recurring | Level 3 | Other debt securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	740	818
Investments	0	0
Recurring | Level 3 | Non-marketable equity securities measured using the measurement alternative
Assets, Fair Value Disclosure [Abstract]
Investments	448	586
Recurring | Level 3 | Trading securities (excluding trading account derivatives)
Assets, Fair Value Disclosure [Abstract]
Trading account assets	3,296	3,548
Recurring | Level 3 | Trading account assets
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	4,211	4,053
Recurring | Level 3 | Trading account assets | Interest rate contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	1,644	1,671
Recurring | Level 3 | Trading account assets | Foreign exchange contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	379	346
Recurring | Level 3 | Trading account assets | Equity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	469	343
Recurring | Level 3 | Trading account assets | Commodity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	1,023	767
Recurring | Level 3 | Trading account assets | Credit derivatives
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	696	926
Recurring | Level 3 | Non-trading derivatives and other financial assets
Assets, Fair Value Disclosure [Abstract]
Other assets, gross	$ 0	$ 0