FAIR VALUE MEASUREMENT - Items Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Assets, Fair Value Disclosure [Abstract]
Federal funds sold and securities borrowed or purchased under agreements to resell, selected portfolios of securities purchased under agreements to resell, Netting	$ (73,476)	$ (44,811)
Trading account assets	251,556	243,925
Netting of cash collateral received	(37,506)	(45,912)
Investments	352,290	353,304
Loans	4,374	3,486
Mortgage servicing rights (MSRs)	558	1,564
Liabilities, Fair Value Disclosure [Abstract]
Federal funds purchased and securities loaned or sold under agreements to repurchase, selected portfolios of securities sold under agreements to repurchase, Netting	(73,476)	(44,811)
Netting of cash collateral paid	(35,659)	(49,811)
Assets transferred from Level 1 to Level 2	4,800	2,600
Assets transferred from Level 2 to Level 1	4,000	4,000
Liabilities transferred from Level 1 to Level 2	400	300
Liabilities transferred from Level 2 to Level 1	300	400
Investments measured at net asset value excluded from Level 3	404	408
Accounting Standards Update 2015-07
Liabilities, Fair Value Disclosure [Abstract]
Investments measured at net asset value excluded from Level 3	400	400
Recurring
Assets, Fair Value Disclosure [Abstract]
Federal funds sold and securities borrowed or purchased under agreements to resell	188,587	173,890
Federal funds sold and securities borrowed or purchased under agreements to resell, selected portfolios of securities purchased under agreements to resell, Netting	(55,638)	(40,686)
Federal funds sold and securities borrowed or purchased under agreements to resell, selected portfolios of securities purchased under agreements to resell	132,949	133,204
Investments	291,716	300,790
Loans	4,374	3,486
Mortgage servicing rights (MSRs)	558	1,564
Assets before netting	1,101,517	1,305,641
Total assets, Netting	(400,571)	(606,943)
Total assets	700,946	698,698
Liabilities, Fair Value Disclosure [Abstract]
Interest-bearing deposits	1,465	1,212
Federal funds purchased and securities loaned or sold under agreements to repurchase, selected portfolios of securities sold under agreements to repurchase, Gross	96,276	74,349
Federal funds purchased and securities loaned or sold under agreements to repurchase, selected portfolios of securities sold under agreements to repurchase, Netting	(55,638)	(40,686)
Federal funds purchased and securities loaned or sold under agreements to repurchase, selected portfolios of securities sold under agreements to repurchase	40,638	33,663
Securities sold, not yet purchased	76,171	80,790
Trading liabilities	77,585	82,618
Short-term borrowings	4,627	2,700
Long-term debt	31,392	26,254
Total liabilities, Gross	614,958	823,220
Total liabilities, Netting	(397,705)	(609,550)
Total liabilities	217,253	213,670
Recurring | Trading derivatives liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	374,226	609,507
Cash collateral received	14,296	15,731
Total trading derivatives and cash collateral, liability	388,522	625,238
Netting agreements	(306,401)	(519,000)
Netting of cash collateral paid	(35,659)	(49,811)
Netting, Liabilities, total of netting agreements and cash collateral received	(342,060)	(568,811)
Total derivative liabilities	46,462	56,427
Cash collateral received, gross	51,802	61,643
Recurring | Trading derivatives liabilities | Interest rate contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	184,429	354,761
Recurring | Trading derivatives liabilities | Foreign exchange contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	120,175	187,761
Recurring | Trading derivatives liabilities | Equity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	31,373	26,516
Recurring | Trading derivatives liabilities | Commodity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	17,027	15,032
Recurring | Trading derivatives liabilities | Credit derivatives
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	21,222	25,437
Recurring | Non-trading derivatives and other financial liabilities
Assets, Fair Value Disclosure [Abstract]
Netting of cash collateral received		(1,345)
Cash collateral paid, gross	7	61
Liabilities, Fair Value Disclosure [Abstract]
Cash collateral received	12	1
Netting of cash collateral paid	(7)	(53)
Netting, Liabilities, total of netting agreements and cash collateral received	(7)	(53)
Other liabilities, gross	15,079	10,848
Non-trading derivatives and other financial liabilities measured on a recurring basis, gross	15,091	10,849
Total other assets and cash collateral, gross	15,084	10,796
Cash collateral received, gross	1,038	1,346
Recurring | Trading account liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	1,414	1,828
Recurring | Mortgage-backed securities - U.S. agency-sponsored
Assets, Fair Value Disclosure [Abstract]
Trading account assets	22,964	22,894
Investments	41,741	38,405
Recurring | Mortgage-backed securities - Residential
Assets, Fair Value Disclosure [Abstract]
Trading account assets	813	690
Investments	2,884	3,910
Recurring | Mortgage-backed securities - Commercial
Assets, Fair Value Disclosure [Abstract]
Trading account assets	1,366	1,206
Investments	332	358
Recurring | Mortgage-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	25,143	24,790
Investments	44,957	42,673
Recurring | U.S. Treasury and federal agency securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	21,137	21,180
Investments	118,146	123,671
Recurring | State and municipal securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	4,700	4,076
Investments	8,765	10,120
Recurring | Foreign government
Assets, Fair Value Disclosure [Abstract]
Trading account assets	60,206	49,696
Investments	100,533	98,148
Recurring | Corporate
Assets, Fair Value Disclosure [Abstract]
Trading account assets	15,705	14,947
Investments	14,109	17,124
Recurring | Equity securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	60,219	50,443
Investments	189	391
Recurring | Asset-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	2,788	2,760
Investments	3,918	6,794
Recurring | Other debt securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	11,723	12,280
Investments	297	503
Recurring | Non-marketable equity securities
Assets, Fair Value Disclosure [Abstract]
Investments	802	1,366
Recurring | Trading securities (excluding trading account derivatives)
Assets, Fair Value Disclosure [Abstract]
Trading account assets	201,621	180,172
Recurring | Trading account assets
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	386,301	617,477
Gross cash collateral paid	7,541	11,188
Trading derivative, asset, gross net cash collateral paid	393,842	628,665
Less: Netting agreements to assets	(306,401)	(519,000)
Netting of cash collateral received	(37,506)	(45,912)
Netting, Assets, total of netting agreements and cash collateral received	(343,907)	(564,912)
Trading derivatives	49,935	63,753
Cash collateral paid, gross	43,200	60,999
Recurring | Trading account assets | Interest rate contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	203,516	369,325
Recurring | Trading account assets | Foreign exchange contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	121,220	185,662
Recurring | Trading account assets | Equity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	26,978	24,110
Recurring | Trading account assets | Commodity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	14,103	13,765
Recurring | Trading account assets | Credit derivatives
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	20,484	24,615
Recurring | Non-trading derivatives and other financial assets
Assets, Fair Value Disclosure [Abstract]
Netting of cash collateral received	(1,026)	(1,345)
Netting, Assets, total of netting agreements and cash collateral received	(1,026)	(1,345)
Other assets, gross	20,819	17,066
Cash collateral paid, gross	0	8
Total other assets and cash collateral, gross	20,819	17,074
Other assets	19,793	15,729
Recurring | Level 1
Assets, Fair Value Disclosure [Abstract]
Federal funds sold and securities borrowed or purchased under agreements to resell	0	0
Investments	165,507	170,495
Loans	0	0
Mortgage servicing rights (MSRs)	0	0
Assets before netting	$ 292,700	$ 276,532
Total as a percentage of gross assets	26.80%	21.40%
Liabilities, Fair Value Disclosure [Abstract]
Interest-bearing deposits	$ 0	$ 0
Federal funds purchased and securities loaned or sold under agreements to repurchase, selected portfolios of securities sold under agreements to repurchase, Gross	0	0
Securities sold, not yet purchased	65,843	67,429
Trading liabilities	65,843	67,429
Short-term borrowings	0	0
Long-term debt	0	0
Total liabilities, Gross	$ 82,437	$ 79,290
Total as a percentage of gross liabilities	13.70%	9.80%
Recurring | Level 1 | Trading derivatives liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	$ 2,691	$ 2,561
Recurring | Level 1 | Trading derivatives liabilities | Interest rate contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	137	107
Recurring | Level 1 | Trading derivatives liabilities | Foreign exchange contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	9	13
Recurring | Level 1 | Trading derivatives liabilities | Equity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	2,430	2,245
Recurring | Level 1 | Trading derivatives liabilities | Commodity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	115	196
Recurring | Level 1 | Trading derivatives 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	13,903	9,300
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 | Mortgage-backed securities - Residential
Assets, Fair Value Disclosure [Abstract]
Trading account assets	0	0
Investments	0	0
Recurring | Level 1 | Mortgage-backed securities - Commercial
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	17,524	16,368
Investments	106,964	112,916
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	39,347	32,164
Investments	56,456	54,028
Recurring | Level 1 | Corporate
Assets, Fair Value Disclosure [Abstract]
Trading account assets	301	424
Investments	1,911	3,215
Recurring | Level 1 | Equity securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	53,305	45,056
Investments	176	336
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	3	0
Investments	0	0
Recurring | Level 1 | Non-marketable equity securities
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	110,480	94,012
Recurring | Level 1 | Trading account assets
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	2,810	2,725
Recurring | Level 1 | Trading account assets | Interest rate contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	145	105
Recurring | Level 1 | Trading account assets | Foreign exchange contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	19	53
Recurring | Level 1 | Trading account assets | Equity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	2,364	2,306
Recurring | Level 1 | Trading account assets | Commodity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	282	261
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	13,903	9,300
Recurring | Level 2
Assets, Fair Value Disclosure [Abstract]
Federal funds sold and securities borrowed or purchased under agreements to resell	188,571	172,394
Investments	123,772	126,434
Loans	3,824	2,918
Mortgage servicing rights (MSRs)	0	0
Assets before netting	$ 790,217	$ 998,387
Total as a percentage of gross assets	72.20%	77.10%
Liabilities, Fair Value Disclosure [Abstract]
Interest-bearing deposits	$ 1,179	$ 919
Federal funds purchased and securities loaned or sold under agreements to repurchase, selected portfolios of securities sold under agreements to repurchase, Gross	95,550	73,500
Securities sold, not yet purchased	10,306	12,184
Trading liabilities	11,715	14,011
Short-term borrowings	4,609	2,658
Long-term debt	18,310	16,510
Total liabilities, Gross	$ 494,879	$ 705,579
Total as a percentage of gross liabilities	82.40%	87.40%
Recurring | Level 2 | Trading derivatives liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	$ 362,348	$ 596,441
Recurring | Level 2 | Trading derivatives liabilities | Interest rate contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	182,162	351,766
Recurring | Level 2 | Trading derivatives liabilities | Foreign exchange contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	119,719	187,328
Recurring | Level 2 | Trading derivatives liabilities | Equity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	26,472	22,119
Recurring | Level 2 | Trading derivatives liabilities | Commodity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	14,482	12,386
Recurring | Level 2 | Trading derivatives liabilities | Credit derivatives
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	19,513	22,842
Recurring | Level 2 | Non-trading derivatives and other financial liabilities
Liabilities, Fair Value Disclosure [Abstract]
Other liabilities, gross	1,168	1,540
Recurring | Level 2 | Trading account liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	1,409	1,827
Recurring | Level 2 | Mortgage-backed securities - U.S. agency-sponsored
Assets, Fair Value Disclosure [Abstract]
Trading account assets	22,801	22,718
Investments	41,717	38,304
Recurring | Level 2 | Mortgage-backed securities - Residential
Assets, Fair Value Disclosure [Abstract]
Trading account assets	649	291
Investments	2,884	3,860
Recurring | Level 2 | Mortgage-backed securities - Commercial
Assets, Fair Value Disclosure [Abstract]
Trading account assets	1,309	1,000
Investments	329	358
Recurring | Level 2 | Mortgage-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	24,759	24,009
Investments	44,930	42,522
Recurring | Level 2 | U.S. Treasury and federal agency securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	3,613	4,811
Investments	11,182	10,753
Recurring | Level 2 | State and municipal securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	4,426	3,780
Investments	8,028	8,909
Recurring | Level 2 | Foreign government
Assets, Fair Value Disclosure [Abstract]
Trading account assets	20,843	17,492
Investments	43,985	43,934
Recurring | Level 2 | Corporate
Assets, Fair Value Disclosure [Abstract]
Trading account assets	15,129	14,199
Investments	12,127	13,598
Recurring | Level 2 | Equity securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	6,794	5,260
Investments	11	46
Recurring | Level 2 | Asset-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	1,198	892
Investments	3,091	6,134
Recurring | Level 2 | Other debt securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	11,105	9,466
Investments	297	503
Recurring | Level 2 | Non-marketable equity securities
Assets, Fair Value Disclosure [Abstract]
Investments	121	35
Recurring | Level 2 | Trading securities (excluding trading account derivatives)
Assets, Fair Value Disclosure [Abstract]
Trading account assets	87,867	79,909
Recurring | Level 2 | Trading account assets
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	379,283	609,000
Recurring | Level 2 | Trading account assets | Interest rate contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	201,663	366,995
Recurring | Level 2 | Trading account assets | Foreign exchange contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	120,624	184,776
Recurring | Level 2 | Trading account assets | Equity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	24,170	21,209
Recurring | Level 2 | Trading account assets | Commodity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	13,252	12,999
Recurring | Level 2 | Trading account assets | Credit derivatives
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	19,574	23,021
Recurring | Level 2 | Non-trading derivatives and other financial assets
Assets, Fair Value Disclosure [Abstract]
Other assets, gross	6,900	7,732
Recurring | Level 3
Assets, Fair Value Disclosure [Abstract]
Federal funds sold and securities borrowed or purchased under agreements to resell	16	1,496
Investments	2,437	3,861
Loans	550	568
Mortgage servicing rights (MSRs)	558	1,564
Assets before netting	$ 11,059	$ 19,526
Total as a percentage of gross assets	1.00%	1.50%
Liabilities, Fair Value Disclosure [Abstract]
Interest-bearing deposits	$ 286	$ 293
Federal funds purchased and securities loaned or sold under agreements to repurchase, selected portfolios of securities sold under agreements to repurchase, Gross	726	849
Securities sold, not yet purchased	22	1,177
Trading liabilities	27	1,178
Short-term borrowings	18	42
Long-term debt	13,082	9,744
Total liabilities, Gross	$ 23,334	$ 22,619
Total as a percentage of gross liabilities	3.90%	2.80%
Recurring | Level 3 | Trading derivatives liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	$ 9,187	$ 10,505
Recurring | Level 3 | Trading derivatives liabilities | Interest rate contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	2,130	2,888
Recurring | Level 3 | Trading derivatives liabilities | Foreign exchange contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	447	420
Recurring | Level 3 | Trading derivatives liabilities | Equity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	2,471	2,152
Recurring | Level 3 | Trading derivatives liabilities | Commodity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	2,430	2,450
Recurring | Level 3 | Trading derivatives liabilities | Credit derivatives
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	1,709	2,595
Recurring | Level 3 | Non-trading derivatives and other financial liabilities
Liabilities, Fair Value Disclosure [Abstract]
Other liabilities, gross	8	8
Recurring | Level 3 | Trading account liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	5	1
Recurring | Level 3 | Mortgage-backed securities - U.S. agency-sponsored
Assets, Fair Value Disclosure [Abstract]
Trading account assets	163	176
Investments	24	101
Recurring | Level 3 | Mortgage-backed securities - Residential
Assets, Fair Value Disclosure [Abstract]
Trading account assets	164	399
Investments	0	50
Recurring | Level 3 | Mortgage-backed securities - Commercial
Assets, Fair Value Disclosure [Abstract]
Trading account assets	57	206
Investments	3	0
Recurring | Level 3 | Mortgage-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	384	781
Investments	27	151
Recurring | Level 3 | U.S. Treasury and federal agency securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	0	1
Investments	0	2
Recurring | Level 3 | State and municipal securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	274	296
Investments	737	1,211
Recurring | Level 3 | Foreign government
Assets, Fair Value Disclosure [Abstract]
Trading account assets	16	40
Investments	92	186
Recurring | Level 3 | Corporate
Assets, Fair Value Disclosure [Abstract]
Trading account assets	275	324
Investments	71	311
Recurring | Level 3 | Equity securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	120	127
Investments	2	9
Recurring | Level 3 | Asset-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	1,590	1,868
Investments	827	660
Recurring | Level 3 | Other debt securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	615	2,814
Investments	0	0
Recurring | Level 3 | Non-marketable equity securities
Assets, Fair Value Disclosure [Abstract]
Investments	681	1,331
Recurring | Level 3 | Trading securities (excluding trading account derivatives)
Assets, Fair Value Disclosure [Abstract]
Trading account assets	3,274	6,251
Recurring | Level 3 | Trading account assets
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	4,208	5,752
Recurring | Level 3 | Trading account assets | Interest rate contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	1,708	2,225
Recurring | Level 3 | Trading account assets | Foreign exchange contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	577	833
Recurring | Level 3 | Trading account assets | Equity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	444	595
Recurring | Level 3 | Trading account assets | Commodity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	569	505
Recurring | Level 3 | Trading account assets | Credit derivatives
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	910	1,594
Recurring | Level 3 | Non-trading derivatives and other financial assets
Assets, Fair Value Disclosure [Abstract]
Other assets, gross	$ 16	$ 34