FAIR VALUE MEASUREMENT - Items Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Assets, Fair Value Disclosure [Abstract]
Federal funds sold and securities borrowed or purchased under agreements to resell, Netting	$ (86,543)	$ (73,476)
Trading account assets	268,808	252,790
Netting of cash collateral received	(40,951)	(38,532)
Investments	351,971	352,290
Loans	4,536	4,374
Mortgage servicing rights	587	558
Liabilities, Fair Value Disclosure [Abstract]
Federal funds purchased and securities loaned or sold under agreements to repurchase, Netting	(86,543)	(73,476)
Netting of cash collateral paid	(33,800)	(35,666)
Assets transferred from Level 1 to Level 2	700	4,800
Assets transferred from Level 2 to Level 1	4,000	4,000
Liabilities transferred from Level 1 to Level 2		400
Liabilities transferred from Level 2 to Level 1	200	300
Investments measured at net asset value excluded from Level 3	433	404
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	222,952	188,587
Federal funds sold and securities borrowed or purchased under agreements to resell, Netting	(61,415)	(55,638)
Federal funds sold and securities borrowed or purchased under agreements to resell	161,537	132,949
Investments	292,659	291,716
Loans	4,536	4,374
Mortgage servicing rights	587	558
Assets before netting	1,154,105	1,101,517
Netting, Assets, total of netting agreements and cash collateral received	(405,535)	(400,571)
Total assets	748,570	700,946
Liabilities, Fair Value Disclosure [Abstract]
Interest-bearing deposits	1,686	1,465
Federal funds purchased and securities loaned or sold under agreements to repurchase, Gross	107,255	96,276
Federal funds purchased and securities loaned or sold under agreements to repurchase, Netting	(61,415)	(55,638)
Federal funds purchased and securities loaned or sold under agreements to repurchase	45,840	40,638
Securities sold, not yet purchased	91,954	76,171
Trading liabilities	94,244	77,585
Short-term borrowings	4,467	4,627
Long-term debt	33,571	31,392
Total liabilities, Gross	643,461	614,958
Total liabilities, Netting	(398,384)	(397,705)
Total liabilities	245,077	217,253
Recurring | Trading account liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	371,715	375,344
Cash collateral received	14,971	14,308
Total trading derivatives and cash collateral, liability	386,686	389,652
Netting agreements	(303,169)	(306,401)
Netting of cash collateral paid	(33,800)	(35,666)
Netting, Liabilities, total of netting agreements and cash collateral received	(336,969)	(342,067)
Total derivative liabilities	49,717	47,585
Cash collateral received, gross	55,922	52,840
Recurring | Trading account liabilities | Interest rate contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	175,586	184,639
Recurring | Trading account liabilities | Foreign exchange contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	123,554	120,772
Recurring | Trading account liabilities | Equity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	34,115	31,373
Recurring | Trading account liabilities | Commodity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	18,851	17,027
Recurring | Trading account liabilities | Credit derivatives
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	19,609	21,533
Recurring | Non-trading derivatives and other financial liabilities
Liabilities, Fair Value Disclosure [Abstract]
Netting of cash collateral paid	(17)	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	15,552	13,961
Total other assets and cash collateral, gross	15,552	13,961
Recurring | Trading account liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	2,290	1,414
Recurring | Mortgage-backed securities - U.S. agency-sponsored
Assets, Fair Value Disclosure [Abstract]
Trading account assets	22,523	22,964
Investments	41,288	41,741
Recurring | Mortgage-backed securities - Residential
Assets, Fair Value Disclosure [Abstract]
Trading account assets	1,227	813
Investments	2,759	2,884
Recurring | Mortgage-backed securities - Commercial
Assets, Fair Value Disclosure [Abstract]
Trading account assets	1,475	1,366
Investments	308	332
Recurring | Mortgage-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	25,225	25,143
Investments	44,355	44,957
Recurring | U.S. Treasury and federal agency securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	24,710	21,137
Investments	117,314	118,146
Recurring | State and municipal securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	3,882	4,700
Investments	9,837	8,765
Recurring | Foreign government
Assets, Fair Value Disclosure [Abstract]
Trading account assets	71,556	60,206
Investments	103,775	100,533
Recurring | Corporate
Assets, Fair Value Disclosure [Abstract]
Trading account assets	19,352	15,705
Investments	12,996	14,109
Recurring | Equity securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	54,287	60,219
Investments	277	189
Recurring | Asset-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	3,183	2,788
Investments	3,081	3,918
Recurring | Other debt securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	11,718	11,723
Investments	165	297
Recurring | Non-marketable equity investments measured using the measurement alternative
Assets, Fair Value Disclosure [Abstract]
Investments	859	802
Recurring | Trading securities (excluding trading account derivatives)
Assets, Fair Value Disclosure [Abstract]
Trading account assets	213,913	201,621
Recurring | Trading account assets
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	390,339	388,561
Cash collateral paid	8,676	7,541
Trading derivative, asset, gross net cash collateral paid	399,015	396,102
Netting agreements	(303,169)	(306,401)
Netting of cash collateral received	(40,951)	(38,532)
Total trading derivatives, netting	(344,120)	(344,933)
Trading derivatives	54,895	51,169
Liabilities, Fair Value Disclosure [Abstract]
Cash collateral paid, gross	42,476	43,207
Recurring | Trading account assets | Interest rate contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	195,829	204,987
Recurring | Trading account assets | Foreign exchange contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	130,217	121,959
Recurring | Trading account assets | Equity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	29,435	26,978
Recurring | Trading account assets | Commodity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	15,831	14,103
Recurring | Trading account assets | Credit derivatives
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	19,027	20,534
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	20,443	18,559
Other assets	20,443	18,559
Recurring | Level 1
Assets, Fair Value Disclosure [Abstract]
Federal funds sold and securities borrowed or purchased under agreements to resell	0	0
Investments	171,540	165,507
Loans	0	0
Mortgage servicing rights	0	0
Assets before netting	$ 305,253	$ 292,700
Total as a percentage of gross assets	26.60%	26.80%
Liabilities, Fair Value Disclosure [Abstract]
Interest-bearing deposits	$ 0	$ 0
Federal funds purchased and securities loaned or sold under agreements to repurchase, Gross	0	0
Securities sold, not yet purchased	80,995	65,843
Trading liabilities	80,995	65,843
Short-term borrowings	0	0
Long-term debt	0	0
Total liabilities, Gross	$ 99,032	$ 82,437
Total as a percentage of gross liabilities	15.80%	13.70%
Recurring | Level 1 | Trading account liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	$ 2,488	$ 2,691
Recurring | Level 1 | Trading account liabilities | Interest rate contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	218	137
Recurring | Level 1 | Trading account liabilities | Foreign exchange contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	4	9
Recurring | Level 1 | Trading account liabilities | Equity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	2,126	2,430
Recurring | Level 1 | Trading account liabilities | Commodity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	138	115
Recurring | Level 1 | Trading account liabilities | Credit derivatives
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	2	0
Recurring | Level 1 | Non-trading derivatives and other financial liabilities
Liabilities, Fair Value Disclosure [Abstract]
Other liabilities, gross	15,549	13,903
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	20,812	17,524
Investments	106,239	106,964
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	46,617	39,347
Investments	61,312	56,456
Recurring | Level 1 | Corporate
Assets, Fair Value Disclosure [Abstract]
Trading account assets	274	301
Investments	3,756	1,911
Recurring | Level 1 | Equity securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	47,797	53,305
Investments	233	176
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	2	3
Investments	0	0
Recurring | Level 1 | Non-marketable equity investments 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	115,502	110,480
Recurring | Level 1 | Trading account assets
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	2,662	2,810
Recurring | Level 1 | Trading account assets | Interest rate contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	276	145
Recurring | Level 1 | Trading account assets | Foreign exchange contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	5	19
Recurring | Level 1 | Trading account assets | Equity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	2,212	2,364
Recurring | Level 1 | Trading account assets | Commodity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	169	282
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	15,549	13,903
Recurring | Level 2
Assets, Fair Value Disclosure [Abstract]
Federal funds sold and securities borrowed or purchased under agreements to resell	222,936	188,571
Investments	119,031	123,772
Loans	3,982	3,824
Mortgage servicing rights	0	0
Assets before netting	$ 828,750	$ 790,217
Total as a percentage of gross assets	72.40%	72.20%
Liabilities, Fair Value Disclosure [Abstract]
Interest-bearing deposits	$ 1,394	$ 1,179
Federal funds purchased and securities loaned or sold under agreements to repurchase, Gross	106,398	95,550
Securities sold, not yet purchased	10,911	10,306
Trading liabilities	13,201	11,715
Short-term borrowings	4,386	4,609
Long-term debt	20,087	18,310
Total liabilities, Gross	$ 505,516	$ 494,879
Total as a percentage of gross liabilities	80.40%	82.40%
Recurring | Level 2 | Trading account liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	$ 360,050	$ 363,466
Recurring | Level 2 | Trading account liabilities | Interest rate contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	173,128	182,372
Recurring | Level 2 | Trading account liabilities | Foreign exchange contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	123,117	120,316
Recurring | Level 2 | Trading account liabilities | Equity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	29,689	26,472
Recurring | Level 2 | Trading account liabilities | Commodity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	16,242	14,482
Recurring | Level 2 | Trading account liabilities | Credit derivatives
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	17,874	19,824
Recurring | Level 2 | Non-trading derivatives and other financial liabilities
Liabilities, Fair Value Disclosure [Abstract]
Other liabilities, gross	0	50
Recurring | Level 2 | Trading account liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	2,290	1,409
Recurring | Level 2 | Mortgage-backed securities - U.S. agency-sponsored
Assets, Fair Value Disclosure [Abstract]
Trading account assets	22,317	22,801
Investments	41,265	41,717
Recurring | Level 2 | Mortgage-backed securities - Residential
Assets, Fair Value Disclosure [Abstract]
Trading account assets	1,084	649
Investments	2,759	2,884
Recurring | Level 2 | Mortgage-backed securities - Commercial
Assets, Fair Value Disclosure [Abstract]
Trading account assets	1,440	1,309
Investments	303	329
Recurring | Level 2 | Mortgage-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	24,841	24,759
Investments	44,327	44,930
Recurring | Level 2 | U.S. Treasury and federal agency securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	3,898	3,613
Investments	11,075	11,182
Recurring | Level 2 | State and municipal securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	3,671	4,426
Investments	9,155	8,028
Recurring | Level 2 | Foreign government
Assets, Fair Value Disclosure [Abstract]
Trading account assets	24,918	20,843
Investments	42,393	43,985
Recurring | Level 2 | Corporate
Assets, Fair Value Disclosure [Abstract]
Trading account assets	18,826	15,129
Investments	9,164	12,127
Recurring | Level 2 | Equity securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	6,253	6,794
Investments	43	11
Recurring | Level 2 | Asset-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	1,586	1,198
Investments	2,584	3,091
Recurring | Level 2 | Other debt securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	11,000	11,105
Investments	165	297
Recurring | Level 2 | Non-marketable equity investments measured using the measurement alternative
Assets, Fair Value Disclosure [Abstract]
Investments	125	121
Recurring | Level 2 | Trading securities (excluding trading account derivatives)
Assets, Fair Value Disclosure [Abstract]
Trading account assets	94,993	87,867
Recurring | Level 2 | Trading account assets
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	382,927	381,543
Recurring | Level 2 | Trading account assets | Interest rate contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	193,319	203,134
Recurring | Level 2 | Trading account assets | Foreign exchange contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	129,691	121,363
Recurring | Level 2 | Trading account assets | Equity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	26,664	24,170
Recurring | Level 2 | Trading account assets | Commodity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	15,100	13,252
Recurring | Level 2 | Trading account assets | Credit derivatives
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	18,153	19,624
Recurring | Level 2 | Non-trading derivatives and other financial assets
Assets, Fair Value Disclosure [Abstract]
Other assets, gross	4,881	4,640
Recurring | Level 3
Assets, Fair Value Disclosure [Abstract]
Federal funds sold and securities borrowed or purchased under agreements to resell	16	16
Investments	2,088	2,437
Loans	554	550
Mortgage servicing rights	587	558
Assets before netting	$ 11,426	$ 11,059
Total as a percentage of gross assets	1.00%	1.00%
Liabilities, Fair Value Disclosure [Abstract]
Interest-bearing deposits	$ 292	$ 286
Federal funds purchased and securities loaned or sold under agreements to repurchase, Gross	857	726
Securities sold, not yet purchased	48	22
Trading liabilities	48	27
Short-term borrowings	81	18
Long-term debt	13,484	13,082
Total liabilities, Gross	$ 23,942	$ 23,334
Total as a percentage of gross liabilities	3.80%	3.90%
Recurring | Level 3 | Trading account liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	$ 9,177	$ 9,187
Recurring | Level 3 | Trading account liabilities | Interest rate contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	2,240	2,130
Recurring | Level 3 | Trading account liabilities | Foreign exchange contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	433	447
Recurring | Level 3 | Trading account liabilities | Equity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	2,300	2,471
Recurring | Level 3 | Trading account liabilities | Commodity contracts
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	2,471	2,430
Recurring | Level 3 | Trading account liabilities | Credit derivatives
Liabilities, Fair Value Disclosure [Abstract]
Trading derivatives, liability	1,733	1,709
Recurring | Level 3 | Non-trading derivatives and other financial liabilities
Liabilities, Fair Value Disclosure [Abstract]
Other liabilities, gross	3	8
Recurring | Level 3 | Trading account liabilities
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	0	5
Recurring | Level 3 | Mortgage-backed securities - U.S. agency-sponsored
Assets, Fair Value Disclosure [Abstract]
Trading account assets	206	163
Investments	23	24
Recurring | Level 3 | Mortgage-backed securities - Residential
Assets, Fair Value Disclosure [Abstract]
Trading account assets	143	164
Investments	0	0
Recurring | Level 3 | Mortgage-backed securities - Commercial
Assets, Fair Value Disclosure [Abstract]
Trading account assets	35	57
Investments	5	3
Recurring | Level 3 | Mortgage-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	384	384
Investments	28	27
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 securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	211	274
Investments	682	737
Recurring | Level 3 | Foreign government
Assets, Fair Value Disclosure [Abstract]
Trading account assets	21	16
Investments	70	92
Recurring | Level 3 | Corporate
Assets, Fair Value Disclosure [Abstract]
Trading account assets	252	275
Investments	76	71
Recurring | Level 3 | Equity securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	237	120
Investments	1	2
Recurring | Level 3 | Asset-backed securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	1,597	1,590
Investments	497	827
Recurring | Level 3 | Other debt securities
Assets, Fair Value Disclosure [Abstract]
Trading account assets	716	615
Investments	0	0
Recurring | Level 3 | Non-marketable equity investments measured using the measurement alternative
Assets, Fair Value Disclosure [Abstract]
Investments	734	681
Recurring | Level 3 | Trading securities (excluding trading account derivatives)
Assets, Fair Value Disclosure [Abstract]
Trading account assets	3,418	3,274
Recurring | Level 3 | Trading account assets
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	4,750	4,208
Recurring | Level 3 | Trading account assets | Interest rate contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	2,234	1,708
Recurring | Level 3 | Trading account assets | Foreign exchange contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	521	577
Recurring | Level 3 | Trading account assets | Equity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	559	444
Recurring | Level 3 | Trading account assets | Commodity contracts
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	562	569
Recurring | Level 3 | Trading account assets | Credit derivatives
Assets, Fair Value Disclosure [Abstract]
Trading derivatives, asset, Gross	874	910
Recurring | Level 3 | Non-trading derivatives and other financial assets
Assets, Fair Value Disclosure [Abstract]
Other assets, gross	$ 13	$ 16