CREDIT QUALITY AND THE ALLOWANCE FOR CREDIT LOSSES - Amortized Cost Basis by Credit Quality Indicator (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Total	$ 143,188		$ 145,959
Financing Receivable, Excluding Accrued Interest, Allowance for Credit Loss, Writeoff, by Origination Year [Abstract]
Year one	4	$ 5
Year two	16	24
Year three	15	45
Year four	24	15
Year five	69	11
Prior to year five	58	23
Within the Revolving Period	44	48
Converted to Term	1	0
Total	231	171
Commercial
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	1,727		5,669
Year two	5,228		13,841
Year three	13,682		12,883
Year four	12,608		4,966
Year five	4,548		4,476
Prior to year five	11,326		8,133
Within the Revolving Period	23,634		24,399
Converted to Term	70		78
Total	72,823		74,445
Financing Receivable, Excluding Accrued Interest, Allowance for Credit Loss, Writeoff, by Origination Year [Abstract]
Year one	0	0
Year two	5	0
Year three	1	27
Year four	4	4
Year five	59	1
Prior to year five	30	3
Within the Revolving Period	3	24
Converted to Term	0	0
Total	102	59
Commercial | Pass
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	1,727		5,600
Year two	5,090		12,303
Year three	11,932		11,393
Year four	11,112		4,000
Year five	3,538		3,441
Prior to year five	8,469		6,194
Within the Revolving Period	22,227		23,008
Converted to Term	52		61
Total	64,147		66,000
Commercial | Special Mention
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	0		59
Year two	62		934
Year three	1,134		894
Year four	855		252
Year five	436		233
Prior to year five	558		373
Within the Revolving Period	441		443
Converted to Term	0		0
Total	3,486		3,188
Commercial | Substandard Accrual
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	0		8
Year two	71		466
Year three	471		540
Year four	576		687
Year five	556		653
Prior to year five	1,717		1,226
Within the Revolving Period	897		892
Converted to Term	11		11
Total	4,299		4,483
Commercial | Nonaccrual
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	0		2
Year two	5		138
Year three	145		56
Year four	65		27
Year five	18		149
Prior to year five	582		340
Within the Revolving Period	69		56
Converted to Term	7		6
Total	891		774
Commercial | Commercial and industrial
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	1,261		3,762
Year two	3,618		6,994
Year three	6,439		6,077
Year four	5,792		1,697
Year five	1,540		1,327
Prior to year five	3,448		2,801
Within the Revolving Period	21,786		22,246
Converted to Term	67		70
Total	43,951		44,974
Financing Receivable, Excluding Accrued Interest, Allowance for Credit Loss, Writeoff, by Origination Year [Abstract]
Year one	0	0
Year two	5	0
Year three	1	27
Year four	4	4
Year five	0	0
Prior to year five	1	0
Within the Revolving Period	3	24
Converted to Term	0	0
Total	14	55
Commercial | Commercial and industrial | Pass
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	1,261		3,694
Year two	3,535		6,512
Year three	5,966		5,331
Year four	5,103		1,445
Year five	1,280		1,147
Prior to year five	2,843		2,299
Within the Revolving Period	20,606		21,033
Converted to Term	49		53
Total	40,643		41,514
Commercial | Commercial and industrial | Special Mention
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	0		59
Year two	56		221
Year three	219		355
Year four	353		30
Year five	66		50
Prior to year five	106		113
Within the Revolving Period	312		368
Converted to Term	0		0
Total	1,112		1,196
Commercial | Commercial and industrial | Substandard Accrual
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	0		8
Year two	22		189
Year three	187		337
Year four	305		218
Year five	188		125
Prior to year five	387		287
Within the Revolving Period	802		792
Converted to Term	11		11
Total	1,902		1,967
Commercial | Commercial and industrial | Nonaccrual
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	0		1
Year two	5		72
Year three	67		54
Year four	31		4
Year five	6		5
Prior to year five	112		102
Within the Revolving Period	66		53
Converted to Term	7		6
Total	294		297
Commercial | Commercial real estate
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	466		1,907
Year two	1,610		6,847
Year three	7,243		6,806
Year four	6,816		3,269
Year five	3,008		3,149
Prior to year five	7,878		5,332
Within the Revolving Period	1,848		2,153
Converted to Term	3		8
Total	28,872		29,471
Financing Receivable, Excluding Accrued Interest, Allowance for Credit Loss, Writeoff, by Origination Year [Abstract]
Year one	0	0
Year two	0	0
Year three	0	0
Year four	0	0
Year five	59	1
Prior to year five	29	3
Within the Revolving Period	0	0
Converted to Term	0	0
Total	88	4
Commercial | Commercial real estate | Pass
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	466		1,906
Year two	1,555		5,791
Year three	5,966		6,062
Year four	6,009		2,555
Year five	2,258		2,294
Prior to year five	5,626		3,895
Within the Revolving Period	1,621		1,975
Converted to Term	3		8
Total	23,504		24,486
Commercial | Commercial real estate | Special Mention
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	0		0
Year two	6		713
Year three	915		539
Year four	502		222
Year five	370		183
Prior to year five	452		260
Within the Revolving Period	129		75
Converted to Term	0		0
Total	2,374		1,992
Commercial | Commercial real estate | Substandard Accrual
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	0		0
Year two	49		277
Year three	284		203
Year four	271		469
Year five	368		528
Prior to year five	1,330		939
Within the Revolving Period	95		100
Converted to Term	0		0
Total	2,397		2,516
Commercial | Commercial real estate | Nonaccrual
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	0		1
Year two	0		66
Year three	78		2
Year four	34		23
Year five	12		144
Prior to year five	470		238
Within the Revolving Period	3		3
Converted to Term	0		0
Total	597		477
Retail
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	873		4,817
Year two	4,817		10,219
Year three	9,895		15,492
Year four	14,866		9,169
Year five	8,766		4,095
Prior to year five	13,105		9,675
Within the Revolving Period	17,010		17,008
Converted to Term	1,033		1,039
Total	70,365		71,514
Financing Receivable, Excluding Accrued Interest, Allowance for Credit Loss, Writeoff, by Origination Year [Abstract]
Year one	4	5
Year two	11	24
Year three	14	18
Year four	20	11
Year five	10	10
Prior to year five	28	20
Within the Revolving Period	41	24
Converted to Term	1	0
Total	129	112
Retail | Residential mortgages
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	548		2,653
Year two	2,656		5,821
Year three	5,816		8,761
Year four	8,673		5,180
Year five	5,110		2,274
Prior to year five	8,709		6,643
Within the Revolving Period	0		0
Converted to Term	0		0
Total	31,512		31,332
Financing Receivable, Excluding Accrued Interest, Allowance for Credit Loss, Writeoff, by Origination Year [Abstract]
Year one	0	0
Year two	0	0
Year three	0	0
Year four	0	0
Year five	0	0
Prior to year five	2	1
Within the Revolving Period	0	0
Converted to Term	0	0
Total	2	1
Retail | Home equity
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	0		1
Year two	1		9
Year three	12		9
Year four	10		7
Year five	7		30
Prior to year five	412		423
Within the Revolving Period	13,644		13,529
Converted to Term	1,027		1,032
Total	15,113		15,040
Financing Receivable, Excluding Accrued Interest, Allowance for Credit Loss, Writeoff, by Origination Year [Abstract]
Year one	0	0
Year two	0	0
Year three	0	0
Year four	0	0
Year five	0	0
Prior to year five	1	1
Within the Revolving Period	2	1
Converted to Term	1	0
Total	4	2
Retail | Automobile
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	0		500
Year two	459		2,335
Year three	2,123		3,444
Year four	3,065		1,207
Year five	1,035		576
Prior to year five	595		196
Within the Revolving Period	0		0
Converted to Term	0		0
Total	7,277		8,258
Financing Receivable, Excluding Accrued Interest, Allowance for Credit Loss, Writeoff, by Origination Year [Abstract]
Year one	0	0
Year two	2	7
Year three	9	11
Year four	11	4
Year five	3	4
Prior to year five	3	4
Within the Revolving Period	0	0
Converted to Term	0	0
Total	28	30
Retail | Education
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	155		852
Year two	1,020		1,737
Year three	1,682		3,114
Year four	2,990		2,622
Year five	2,495		1,150
Prior to year five	3,304		2,359
Within the Revolving Period	0		0
Converted to Term	0		0
Total	11,646		11,834
Financing Receivable, Excluding Accrued Interest, Allowance for Credit Loss, Writeoff, by Origination Year [Abstract]
Year one	0	0
Year two	0	2
Year three	1	3
Year four	6	4
Year five	7	3
Prior to year five	18	11
Within the Revolving Period	0	0
Converted to Term	0	0
Total	32	23
Retail | Other retail
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	170		811
Year two	681		317
Year three	262		164
Year four	128		153
Year five	119		65
Prior to year five	85		54
Within the Revolving Period	3,366		3,479
Converted to Term	6		7
Total	4,817		5,050
Financing Receivable, Excluding Accrued Interest, Allowance for Credit Loss, Writeoff, by Origination Year [Abstract]
Year one	4	5
Year two	9	15
Year three	4	4
Year four	3	3
Year five	0	3
Prior to year five	4	3
Within the Revolving Period	39	23
Converted to Term	0	0
Total	63	$ 56
Greater than 800 | Retail
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	245		1,449
Year two	1,762		4,351
Year three	4,404		7,913
Year four	7,810		4,939
Year five	4,763		1,913
Prior to year five	6,184		4,466
Within the Revolving Period	5,421		5,578
Converted to Term	212		222
Total	30,801		30,831
Greater than 800 | Retail | Residential mortgages
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	152		889
Year two	1,165		3,067
Year three	3,188		5,172
Year four	5,210		3,117
Year five	3,050		1,131
Prior to year five	4,217		3,125
Within the Revolving Period	0		0
Converted to Term	0		0
Total	16,982		16,501
Greater than 800 | Retail | Home equity
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	0		0
Year two	0		4
Year three	4		4
Year four	4		1
Year five	1		4
Prior to year five	89		91
Within the Revolving Period	4,938		5,078
Converted to Term	212		222
Total	5,248		5,404
Greater than 800 | Retail | Automobile
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	0		81
Year two	73		539
Year three	483		1,062
Year four	936		368
Year five	309		162
Prior to year five	155		47
Within the Revolving Period	0		0
Converted to Term	0		0
Total	1,956		2,259
Greater than 800 | Retail | Education
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	55		296
Year two	373		671
Year three	673		1,637
Year four	1,630		1,418
Year five	1,376		600
Prior to year five	1,699		1,185
Within the Revolving Period	0		0
Converted to Term	0		0
Total	5,806		5,807
Greater than 800 | Retail | Other retail
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	38		183
Year two	151		70
Year three	56		38
Year four	30		35
Year five	27		16
Prior to year five	24		18
Within the Revolving Period	483		500
Converted to Term	0		0
Total	809		860
740-799 | Retail
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	423		2,093
Year two	1,847		3,393
Year three	3,166		4,696
Year four	4,338		2,682
Year five	2,562		1,150
Prior to year five	3,350		2,456
Within the Revolving Period	5,746		5,671
Converted to Term	240		242
Total	21,672		22,383
740-799 | Retail | Residential mortgages
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	299		1,333
Year two	1,099		1,940
Year three	1,827		2,560
Year four	2,421		1,411
Year five	1,420		592
Prior to year five	2,111		1,625
Within the Revolving Period	0		0
Converted to Term	0		0
Total	9,177		9,461
740-799 | Retail | Home equity
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	0		0
Year two	1		1
Year three	2		2
Year four	2		1
Year five	1		3
Prior to year five	76		82
Within the Revolving Period	4,822		4,708
Converted to Term	239		241
Total	5,143		5,038
740-799 | Retail | Automobile
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	0		134
Year two	123		671
Year three	609		1,038
Year four	919		375
Year five	322		165
Prior to year five	166		52
Within the Revolving Period	0		0
Converted to Term	0		0
Total	2,139		2,435
740-799 | Retail | Education
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	69		368
Year two	413		694
Year three	658		1,050
Year four	961		850
Year five	784		369
Prior to year five	968		678
Within the Revolving Period	0		0
Converted to Term	0		0
Total	3,853		4,009
740-799 | Retail | Other retail
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	55		258
Year two	211		87
Year three	70		46
Year four	35		45
Year five	35		21
Prior to year five	29		19
Within the Revolving Period	924		963
Converted to Term	1		1
Total	1,360		1,440
680-739 | Retail
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	160		872
Year two	803		1,574
Year three	1,479		1,839
Year four	1,717		1,032
Year five	963		541
Prior to year five	1,711		1,314
Within the Revolving Period	3,747		3,666
Converted to Term	194		204
Total	10,774		11,042
680-739 | Retail | Residential mortgages
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	92		367
Year two	317		631
Year three	630		758
Year four	754		466
Year five	462		266
Prior to year five	1,071		873
Within the Revolving Period	0		0
Converted to Term	0		0
Total	3,326		3,361
680-739 | Retail | Home equity
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	0		1
Year two	0		1
Year three	1		1
Year four	1		2
Year five	2		5
Prior to year five	92		93
Within the Revolving Period	2,822		2,693
Converted to Term	192		202
Total	3,110		2,998
680-739 | Retail | Automobile
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	0		147
Year two	134		577
Year three	520		708
Year four	627		252
Year five	215		118
Prior to year five	122		39
Within the Revolving Period	0		0
Converted to Term	0		0
Total	1,618		1,841
680-739 | Retail | Education
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	22		143
Year two	175		289
Year three	267		333
Year four	305		273
Year five	254		134
Prior to year five	405		298
Within the Revolving Period	0		0
Converted to Term	0		0
Total	1,428		1,470
680-739 | Retail | Other retail
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	46		214
Year two	177		76
Year three	61		39
Year four	30		39
Year five	30		18
Prior to year five	21		11
Within the Revolving Period	925		973
Converted to Term	2		2
Total	1,292		1,372
620-679 | Retail
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	35		296
Year two	289		565
Year three	509		602
Year four	564		281
Year five	261		232
Prior to year five	842		659
Within the Revolving Period	1,128		1,137
Converted to Term	146		139
Total	3,774		3,911
620-679 | Retail | Residential mortgages
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	5		54
Year two	57		135
Year three	116		165
Year four	168		90
Year five	90		121
Prior to year five	557		445
Within the Revolving Period	0		0
Converted to Term	0		0
Total	993		1,010
620-679 | Retail | Home equity
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	0		0
Year two	0		1
Year three	2		1
Year four	1		2
Year five	2		8
Prior to year five	76		77
Within the Revolving Period	731		718
Converted to Term	144		137
Total	956		944
620-679 | Retail | Automobile
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	0		94
Year two	83		316
Year three	287		345
Year four	309		112
Year five	98		65
Prior to year five	72		26
Within the Revolving Period	0		0
Converted to Term	0		0
Total	849		958
620-679 | Retail | Education
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	6		30
Year two	50		65
Year three	64		68
Year four	68		58
Year five	57		32
Prior to year five	130		107
Within the Revolving Period	0		0
Converted to Term	0		0
Total	375		360
620-679 | Retail | Other retail
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	24		118
Year two	99		48
Year three	40		23
Year four	18		19
Year five	14		6
Prior to year five	7		4
Within the Revolving Period	397		419
Converted to Term	2		2
Total	601		639
Less than 620 | Retail
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	4		89
Year two	112		335
Year three	337		439
Year four	435		233
Year five	214		256
Prior to year five	967		730
Within the Revolving Period	578		583
Converted to Term	241		232
Total	2,888		2,897
Less than 620 | Retail | Residential mortgages
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	0		9
Year two	18		48
Year three	55		104
Year four	118		95
Year five	87		161
Prior to year five	736		561
Within the Revolving Period	0		0
Converted to Term	0		0
Total	1,014		978
Less than 620 | Retail | Home equity
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	0		0
Year two	0		2
Year three	3		1
Year four	2		1
Year five	1		10
Prior to year five	79		80
Within the Revolving Period	331		332
Converted to Term	240		230
Total	656		656
Less than 620 | Retail | Automobile
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	0		44
Year two	46		232
Year three	224		291
Year four	274		100
Year five	91		66
Prior to year five	80		32
Within the Revolving Period	0		0
Converted to Term	0		0
Total	715		765
Less than 620 | Retail | Education
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	1		5
Year two	8		18
Year three	20		25
Year four	26		23
Year five	24		15
Prior to year five	68		55
Within the Revolving Period	0		0
Converted to Term	0		0
Total	147		141
Less than 620 | Retail | Other retail
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	3		31
Year two	40		35
Year three	35		18
Year four	15		14
Year five	11		4
Prior to year five	4		2
Within the Revolving Period	247		251
Converted to Term	1		2
Total	356		357
No FICO available | Retail
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	6		18
Year two	4		1
Year three	0		3
Year four	2		2
Year five	3		3
Prior to year five	51		50
Within the Revolving Period	390		373
Converted to Term	0		0
Total	456		450
No FICO available | Retail | Residential mortgages
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	0		1
Year two	0		0
Year three	0		2
Year four	2		1
Year five	1		3
Prior to year five	17		14
Within the Revolving Period	0		0
Converted to Term	0		0
Total	20		21
No FICO available | Retail | Education
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	2		10
Year two	1		0
Year three	0		1
Year four	0		0
Year five	0		0
Prior to year five	34		36
Within the Revolving Period	0		0
Converted to Term	0		0
Total	37		47
No FICO available | Retail | Other retail
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, by Origination Year [Abstract]
Year one	4		7
Year two	3		1
Year three	0		0
Year four	0		1
Year five	2		0
Prior to year five	0		0
Within the Revolving Period	390		373
Converted to Term	0		0
Total	$ 399		$ 382