FAIR VALUE MEASUREMENT - Items Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Assets, Fair Value Disclosure [Abstract]
Federal funds sold and securities borrowed and purchased under agreements to resell, Netting	$ (104,713)	$ (87,424)
Trading account assets	286,511	256,117
Netting of cash collateral received	(39,750)	(38,608)
Investments	349,281	358,607
Loans at fair value	3,874	3,223
Liabilities, Fair Value Disclosure [Abstract]
Federal funds purchased and securities loaned and sold under agreements to repurchase, Netting	(104,713)	(87,424)
Netting of cash collateral paid	(33,190)	(29,911)
Fair value
Liabilities, Fair Value Disclosure [Abstract]
Investments measured at net asset value excluded from Level 3	199	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	241,480	214,685
Federal funds sold and securities borrowed and purchased under agreements to resell, Netting	(79,364)	(66,984)
Federal funds sold and securities borrowed and purchased under agreements to resell	162,116	147,701
Investments	275,945	288,940
Loans at fair value	3,874	3,223
Mortgage servicing rights	551	584
Assets before netting	1,160,050	1,134,034
Netting, Assets, total of netting agreements and cash collateral received	(411,235)	(416,681)
Total assets	748,815	717,353
Liabilities, Fair Value Disclosure [Abstract]
Interest-bearing deposits	2,344	1,475
Federal funds purchased and securities loaned and sold under agreements to repurchase, Gross	125,605	111,494
Federal funds purchased and securities loaned and sold under agreements to repurchase, Netting	(79,364)	(66,984)
Federal funds purchased and securities loaned and sold under agreements to repurchase	46,241	44,510
Securities sold, not yet purchased	87,887	90,822
Trading liabilities	88,004	92,369
Short-term borrowings	5,172	4,483
Long-term debt	44,088	38,229
Total liabilities, Gross	653,489	656,892
Total liabilities, Netting	(404,675)	(407,984)
Total liabilities	248,814	248,908
Recurring | Trading account liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	359,813	379,030
Cash collateral received	13,886	13,906
Total trading derivatives and cash collateral, liability	373,699	392,936
Netting agreements	(292,121)	(311,089)
Netting of cash collateral paid	(33,190)	(29,911)
Netting, Liabilities, total of netting agreements and cash collateral received	(325,311)	(341,000)
Total derivative liabilities	48,388	51,936
Cash collateral received, gross	53,636	52,514
Recurring | Trading account liabilities | Interest rate contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	166,914	154,827
Recurring | Trading account liabilities | Foreign exchange contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	132,108	159,355
Recurring | Trading account liabilities | Equity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	31,628	33,808
Recurring | Trading account liabilities | Commodity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	17,678	20,689
Recurring | Trading account liabilities | Credit derivatives
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	11,485	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	14,577	15,906
Total other assets and cash collateral, gross	14,577	15,906
Recurring | Trading account liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	117	1,547
Recurring | Mortgage-backed securities - U.S. agency-sponsored
Assets, Fair Value Disclosure [Abstract]
Trading account assets	26,439	24,246
Investments	39,333	43,020
Recurring | Residential mortgage
Assets, Fair Value Disclosure [Abstract]
Trading account assets	702	977
Investments	1,121	1,313
Recurring | Commercial and other
Assets, Fair Value Disclosure [Abstract]
Trading account assets	1,428	1,400
Investments	138	172
Recurring | Mortgage-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	28,569	26,623
Investments	40,592	44,505
Recurring | U.S. Treasury and federal agency securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	41,545	31,242
Investments	107,673	117,222
Recurring | State and municipal securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	2,603	3,982
Investments	8,109	9,206
Recurring | Foreign government
Assets, Fair Value Disclosure [Abstract]
Trading account assets	75,123	64,519
Investments	101,356	100,691
Recurring | Corporate
Assets, Fair Value Disclosure [Abstract]
Trading account assets	16,698	15,896
Investments	12,310	11,599
Recurring | Equity securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	55,920	43,803
Investments	208	220
Recurring | Asset-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	3,007	2,913
Investments	1,420	843
Recurring | Other debt securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	11,632	13,019
Investments	3,672	3,972
Recurring | Non-marketable equity securities measured using the measurement alternative
Assets, Fair Value Disclosure [Abstract]
Investments	605	682
Recurring | Trading securities (excluding trading account derivatives)
Assets, Fair Value Disclosure [Abstract]
Trading account assets	235,097	201,997
Recurring | Trading account assets
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	371,936	392,299
Cash collateral paid	11,349	11,518
Trading derivative, asset, gross net cash collateral paid	383,285	403,817
Netting agreements	(292,121)	(311,089)
Netting of cash collateral received	(39,750)	(38,608)
Total trading derivatives, netting	(331,871)	(349,697)
Trading derivatives	51,414	54,120
Liabilities, Fair Value Disclosure [Abstract]
Cash collateral paid, gross	44,539	41,429
Recurring | Trading account assets | Interest rate contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	186,123	171,632
Recurring | Trading account assets | Foreign exchange contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	134,366	162,454
Recurring | Trading account assets | Equity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	25,783	29,893
Recurring | Trading account assets | Commodity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	14,980	17,555
Recurring | Trading account assets | Credit derivatives
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	10,684	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	19,818	20,788
Other assets	19,818	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	164,404	170,445
Loans at fair value	0	0
Mortgage servicing rights	0	0
Assets before netting	$ 317,257	$ 294,151
Total as a percentage of gross assets	27.60%	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	74,650	78,872
Trading liabilities	74,650	78,872
Short-term borrowings	0	0
Long-term debt	0	0
Total liabilities, Gross	$ 89,616	$ 95,133
Total as a percentage of gross liabilities	14.00%	14.80%
Recurring | Level 1 | Trading account liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	$ 389	$ 422
Recurring | Level 1 | Trading account liabilities | Interest rate contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	179	71
Recurring | Level 1 | Trading account liabilities | Foreign exchange contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	3	0
Recurring | Level 1 | Trading account liabilities | Equity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	207	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	14,577	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	36,579	26,439
Investments	98,793	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	51,976	43,309
Investments	60,695	58,252
Recurring | Level 1 | Corporate
Assets, Fair Value Disclosure [Abstract]
Trading account assets	1,949	1,026
Investments	4,722	4,410
Recurring | Level 1 | Equity securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	46,995	36,342
Investments	194	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	40	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	137,539	107,119
Recurring | Level 1 | Trading account assets
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	737	748
Recurring | Level 1 | Trading account assets | Interest rate contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	283	101
Recurring | Level 1 | Trading account assets | Foreign exchange contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	1	0
Recurring | Level 1 | Trading account assets | Equity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	453	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	14,577	15,839
Recurring | Level 2
Assets, Fair Value Disclosure [Abstract]
Federal funds sold and securities borrowed and purchased under agreements to resell	241,414	214,570
Investments	109,157	116,758
Loans at fair value	3,501	2,946
Mortgage servicing rights	0	0
Assets before netting	$ 820,765	$ 818,051
Total as a percentage of gross assets	71.50%	72.90%
Liabilities, Fair Value Disclosure [Abstract]
Interest-bearing deposits	$ 1,297	$ 980
Federal funds purchased and securities loaned and sold under agreements to repurchase, Gross	124,564	110,511
Securities sold, not yet purchased	13,222	11,364
Trading liabilities	13,339	12,911
Short-term borrowings	5,002	4,446
Long-term debt	30,354	25,659
Total liabilities, Gross	$ 529,142	$ 528,228
Total as a percentage of gross liabilities	82.70%	82.10%
Recurring | Level 2 | Trading account liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	$ 354,586	$ 373,654
Recurring | Level 2 | Trading account liabilities | Interest rate contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	165,038	152,931
Recurring | Level 2 | Trading account liabilities | Foreign exchange contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	131,797	159,003
Recurring | Level 2 | Trading account liabilities | Equity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	29,734	32,330
Recurring | Level 2 | Trading account liabilities | Commodity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	17,248	19,904
Recurring | Level 2 | Trading account liabilities | Credit derivatives
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	10,769	9,486
Recurring | Level 2 | Non-trading derivatives and other financial liabilities
Liabilities, Fair Value Disclosure [Abstract]
Other liabilities, gross	0	67
Recurring | Level 2 | Trading account liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	117	1,547
Recurring | Level 2 | Mortgage-backed securities - U.S. agency-sponsored
Assets, Fair Value Disclosure [Abstract]
Trading account assets	26,285	24,090
Investments	39,301	42,988
Recurring | Level 2 | Residential mortgage
Assets, Fair Value Disclosure [Abstract]
Trading account assets	574	709
Investments	1,121	1,313
Recurring | Level 2 | Commercial and other
Assets, Fair Value Disclosure [Abstract]
Trading account assets	1,359	1,323
Investments	138	172
Recurring | Level 2 | Mortgage-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	28,218	26,122
Investments	40,560	44,473
Recurring | Level 2 | U.S. Treasury and federal agency securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	4,966	4,802
Investments	8,880	9,645
Recurring | Level 2 | State and municipal securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	2,425	3,782
Investments	7,199	8,498
Recurring | Level 2 | Foreign government
Assets, Fair Value Disclosure [Abstract]
Trading account assets	23,108	21,179
Investments	40,590	42,371
Recurring | Level 2 | Corporate
Assets, Fair Value Disclosure [Abstract]
Trading account assets	14,371	14,510
Investments	7,528	7,033
Recurring | Level 2 | Equity securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	8,798	7,308
Investments	14	14
Recurring | Level 2 | Asset-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	1,578	1,429
Investments	614	656
Recurring | Level 2 | Other debt securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	10,550	12,198
Investments	3,672	3,972
Recurring | Level 2 | Non-marketable equity securities measured using the measurement alternative
Assets, Fair Value Disclosure [Abstract]
Investments	100	96
Recurring | Level 2 | Trading securities (excluding trading account derivatives)
Assets, Fair Value Disclosure [Abstract]
Trading account assets	94,014	91,330
Recurring | Level 2 | Trading account assets
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	367,438	387,498
Recurring | Level 2 | Trading account assets | Interest rate contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	184,259	169,860
Recurring | Level 2 | Trading account assets | Foreign exchange contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	134,011	162,108
Recurring | Level 2 | Trading account assets | Equity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	24,988	28,903
Recurring | Level 2 | Trading account assets | Commodity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	14,246	16,788
Recurring | Level 2 | Trading account assets | Credit derivatives
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	9,934	9,839
Recurring | Level 2 | Non-trading derivatives and other financial assets
Assets, Fair Value Disclosure [Abstract]
Other assets, gross	5,241	4,949
Recurring | Level 3
Assets, Fair Value Disclosure [Abstract]
Federal funds sold and securities borrowed and purchased under agreements to resell	66	115
Investments	2,384	1,737
Loans at fair value	373	277
Mortgage servicing rights	551	584
Assets before netting	$ 10,679	$ 10,314
Total as a percentage of gross assets	0.90%	0.90%
Liabilities, Fair Value Disclosure [Abstract]
Interest-bearing deposits	$ 1,047	$ 495
Federal funds purchased and securities loaned and sold under agreements to repurchase, Gross	1,041	983
Securities sold, not yet purchased	15	586
Trading liabilities	15	586
Short-term borrowings	170	37
Long-term debt	13,734	12,570
Total liabilities, Gross	$ 20,845	$ 19,625
Total as a percentage of gross liabilities	3.30%	3.10%
Recurring | Level 3 | Trading account liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	$ 4,838	$ 4,954
Recurring | Level 3 | Trading account liabilities | Interest rate contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	1,697	1,825
Recurring | Level 3 | Trading account liabilities | Foreign exchange contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	308	352
Recurring | Level 3 | Trading account liabilities | Equity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	1,687	1,127
Recurring | Level 3 | Trading account liabilities | Commodity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	430	785
Recurring | Level 3 | Trading account liabilities | Credit derivatives
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	716	865
Recurring | Level 3 | Non-trading derivatives and other financial liabilities
Liabilities, Fair Value Disclosure [Abstract]
Other liabilities, gross	0	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	154	156
Investments	32	32
Recurring | Level 3 | Residential mortgage
Assets, Fair Value Disclosure [Abstract]
Trading account assets	128	268
Investments	0	0
Recurring | Level 3 | Commercial and other
Assets, Fair Value Disclosure [Abstract]
Trading account assets	69	77
Investments	0	0
Recurring | Level 3 | Mortgage-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	351	501
Investments	32	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	178	200
Investments	910	708
Recurring | Level 3 | Foreign government
Assets, Fair Value Disclosure [Abstract]
Trading account assets	39	31
Investments	71	68
Recurring | Level 3 | Corporate
Assets, Fair Value Disclosure [Abstract]
Trading account assets	378	360
Investments	60	156
Recurring | Level 3 | Equity securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	127	153
Investments	0	0
Recurring | Level 3 | Asset-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	1,429	1,484
Investments	806	187
Recurring | Level 3 | Other debt securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	1,042	818
Investments	0	0
Recurring | Level 3 | Non-marketable equity securities measured using the measurement alternative
Assets, Fair Value Disclosure [Abstract]
Investments	505	586
Recurring | Level 3 | Trading securities (excluding trading account derivatives)
Assets, Fair Value Disclosure [Abstract]
Trading account assets	3,544	3,548
Recurring | Level 3 | Trading account assets
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	3,761	4,053
Recurring | Level 3 | Trading account assets | Interest rate contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	1,581	1,671
Recurring | Level 3 | Trading account assets | Foreign exchange contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	354	346
Recurring | Level 3 | Trading account assets | Equity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	342	343
Recurring | Level 3 | Trading account assets | Commodity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	734	767
Recurring | Level 3 | Trading account assets | Credit derivatives
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	750	926
Recurring | Level 3 | Non-trading derivatives and other financial assets
Assets, Fair Value Disclosure [Abstract]
Other assets, gross	$ 0	$ 0