ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Amortized Cost Basis by Credit Quality Indicator (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
Total	$ 124,071	$ 119,088
Commercial
Financing Receivable, Credit Quality Indicator [Line Items]
2020	9,853
2019	11,332
2018	9,674
2017	5,060
2016	3,009
Prior to 2016	5,000
Within the Revolving Period	18,057
Converted to Term	377
Total	62,362	57,538
Commercial | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
2020	9,584
2019	10,556
2018	8,711
2017	4,469
2016	2,540
Prior to 2016	4,482
Within the Revolving Period	16,188
Converted to Term	182
Total	56,712
Commercial | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
2020	69
2019	493
2018	368
2017	392
2016	268
Prior to 2016	256
Within the Revolving Period	969
Converted to Term	125
Total	2,940
Commercial | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
2020	128
2019	223
2018	498
2017	155
2016	142
Prior to 2016	206
Within the Revolving Period	690
Converted to Term	19
Total	2,061
Commercial | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
2020	72
2019	60
2018	97
2017	44
2016	59
Prior to 2016	56
Within the Revolving Period	210
Converted to Term	51
Total	649
Commercial | Commercial
Financing Receivable, Credit Quality Indicator [Line Items]
2020	7,648
2019	7,340
2018	5,436
2017	3,039
2016	1,712
Prior to 2016	2,857
Within the Revolving Period	16,823
Converted to Term	330
Total	45,185	41,479
Commercial | Commercial | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
2020	7,478
2019	6,823
2018	4,838
2017	2,747
2016	1,515
Prior to 2016	2,453
Within the Revolving Period	15,115
Converted to Term	182
Total	41,151
Commercial | Commercial | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
2020	58
2019	275
2018	237
2017	149
2016	92
Prior to 2016	222
Within the Revolving Period	892
Converted to Term	125
Total	2,050
Commercial | Commercial | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
2020	60
2019	220
2018	326
2017	100
2016	85
Prior to 2016	140
Within the Revolving Period	630
Converted to Term	19
Total	1,580
Commercial | Commercial | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
2020	52
2019	22
2018	35
2017	43
2016	20
Prior to 2016	42
Within the Revolving Period	186
Converted to Term	4
Total	404
Commercial | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1,873
2019	3,661
2018	3,959
2017	1,840
2016	1,085
Prior to 2016	1,190
Within the Revolving Period	1,234
Converted to Term	47
Total	14,889	13,522
Commercial | Commercial real estate | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1,774
2019	3,406
2018	3,608
2017	1,553
2016	825
Prior to 2016	1,112
Within the Revolving Period	1,073
Converted to Term	0
Total	13,351
Commercial | Commercial real estate | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
2020	11
2019	216
2018	128
2017	237
2016	171
Prior to 2016	9
Within the Revolving Period	77
Converted to Term	0
Total	849
Commercial | Commercial real estate | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
2020	68
2019	1
2018	170
2017	50
2016	53
Prior to 2016	66
Within the Revolving Period	60
Converted to Term	0
Total	468
Commercial | Commercial real estate | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
2020	20
2019	38
2018	53
2017	0
2016	36
Prior to 2016	3
Within the Revolving Period	24
Converted to Term	47
Total	221
Commercial | Leases
Financing Receivable, Credit Quality Indicator [Line Items]
2020	332
2019	331
2018	279
2017	181
2016	212
Prior to 2016	953
Within the Revolving Period	0
Converted to Term	0
Total	2,288	2,537
Commercial | Leases | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
2020	332
2019	327
2018	265
2017	169
2016	200
Prior to 2016	917
Within the Revolving Period	0
Converted to Term	0
Total	2,210
Commercial | Leases | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	2
2018	3
2017	6
2016	5
Prior to 2016	25
Within the Revolving Period	0
Converted to Term	0
Total	41
Commercial | Leases | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	2
2018	2
2017	5
2016	4
Prior to 2016	0
Within the Revolving Period	0
Converted to Term	0
Total	13
Commercial | Leases | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	9
2017	1
2016	3
Prior to 2016	11
Within the Revolving Period	0
Converted to Term	0
Total	24
Commercial | SBA Guaranteed PPP Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total	4,700
Retail
Financing Receivable, Credit Quality Indicator [Line Items]
2020	12,825
2019	12,260
2018	5,972
2017	5,429
2016	4,976
Prior to 2016	7,044
Within the Revolving Period	11,770
Converted to Term	1,433
Total	61,709	61,550
Retail | Residential mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
2020	5,040
2019	3,853
2018	1,485
2017	2,224
2016	3,049
Prior to 2016	3,982
Within the Revolving Period	0
Converted to Term	0
Total	19,633	19,083
Retail | Home equity
Financing Receivable, Credit Quality Indicator [Line Items]
2020	5
2019	43
2018	70
2017	77
2016	47
Prior to 2016	921
Within the Revolving Period	9,752
Converted to Term	1,407
Total	12,322	13,154
Retail | Automobile
Financing Receivable, Credit Quality Indicator [Line Items]
2020	3,552
2019	3,804
2018	2,107
2017	1,433
2016	791
Prior to 2016	348
Within the Revolving Period	0
Converted to Term	0
Total	12,035	12,120
Retail | Education
Financing Receivable, Credit Quality Indicator [Line Items]
2020	2,918
2019	2,864
2018	1,703
2017	1,431
2016	1,031
Prior to 2016	1,684
Within the Revolving Period	0
Converted to Term	0
Total	11,631	10,347
Retail | Other retail
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1,310
2019	1,696
2018	607
2017	264
2016	58
Prior to 2016	109
Within the Revolving Period	2,018
Converted to Term	26
Total	6,088	$ 6,846
Greater than 800 | Retail
Financing Receivable, Credit Quality Indicator [Line Items]
2020	4,231
2019	4,604
2018	2,218
2017	2,513
2016	2,650
Prior to 2016	3,222
Within the Revolving Period	4,632
Converted to Term	360
Total	24,430
Greater than 800 | Retail | Residential mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1,971
2019	1,983
2018	751
2017	1,297
2016	1,833
Prior to 2016	2,061
Within the Revolving Period	0
Converted to Term	0
Total	9,896
Greater than 800 | Retail | Home equity
Financing Receivable, Credit Quality Indicator [Line Items]
2020	3
2019	9
2018	11
2017	7
2016	5
Prior to 2016	245
Within the Revolving Period	4,319
Converted to Term	360
Total	4,959
Greater than 800 | Retail | Automobile
Financing Receivable, Credit Quality Indicator [Line Items]
2020	780
2019	862
2018	472
2017	358
2016	209
Prior to 2016	91
Within the Revolving Period	0
Converted to Term	0
Total	2,772
Greater than 800 | Retail | Education
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1,191
2019	1,276
2018	810
2017	772
2016	586
Prior to 2016	776
Within the Revolving Period	0
Converted to Term	0
Total	5,411
Greater than 800 | Retail | Other retail
Financing Receivable, Credit Quality Indicator [Line Items]
2020	286
2019	474
2018	174
2017	79
2016	17
Prior to 2016	49
Within the Revolving Period	313
Converted to Term	0
Total	1,392
740-799 | Retail
Financing Receivable, Credit Quality Indicator [Line Items]
2020	5,208
2019	4,237
2018	1,914
2017	1,571
2016	1,309
Prior to 2016	1,744
Within the Revolving Period	3,845
Converted to Term	342
Total	20,170
740-799 | Retail | Residential mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
2020	2,351
2019	1,341
2018	463
2017	612
2016	776
Prior to 2016	977
Within the Revolving Period	0
Converted to Term	0
Total	6,520
740-799 | Retail | Home equity
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1
2019	7
2018	7
2017	7
2016	4
Prior to 2016	204
Within the Revolving Period	3,224
Converted to Term	340
Total	3,794
740-799 | Retail | Automobile
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1,130
2019	1,127
2018	606
2017	406
2016	214
Prior to 2016	87
Within the Revolving Period	0
Converted to Term	0
Total	3,570
740-799 | Retail | Education
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1,307
2019	1,151
2018	621
2017	449
2016	292
Prior to 2016	443
Within the Revolving Period	0
Converted to Term	0
Total	4,263
740-799 | Retail | Other retail
Financing Receivable, Credit Quality Indicator [Line Items]
2020	419
2019	611
2018	217
2017	97
2016	23
Prior to 2016	33
Within the Revolving Period	621
Converted to Term	2
Total	2,023
680-739 | Retail
Financing Receivable, Credit Quality Indicator [Line Items]
2020	2,365
2019	2,211
2018	1,087
2017	768
2016	618
Prior to 2016	1,022
Within the Revolving Period	2,226
Converted to Term	299
Total	10,596
680-739 | Retail | Residential mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
2020	596
2019	408
2018	193
2017	207
2016	317
Prior to 2016	500
Within the Revolving Period	0
Converted to Term	0
Total	2,221
680-739 | Retail | Home equity
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	4
2018	9
2017	14
2016	8
Prior to 2016	197
Within the Revolving Period	1,671
Converted to Term	293
Total	2,196
680-739 | Retail | Automobile
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1,028
2019	1,012
2018	527
2017	327
2016	170
Prior to 2016	69
Within the Revolving Period	0
Converted to Term	0
Total	3,133
680-739 | Retail | Education
Financing Receivable, Credit Quality Indicator [Line Items]
2020	385
2019	378
2018	217
2017	160
2016	110
Prior to 2016	239
Within the Revolving Period	0
Converted to Term	0
Total	1,489
680-739 | Retail | Other retail
Financing Receivable, Credit Quality Indicator [Line Items]
2020	356
2019	409
2018	141
2017	60
2016	13
Prior to 2016	17
Within the Revolving Period	555
Converted to Term	6
Total	1,557
620-679 | Retail
Financing Receivable, Credit Quality Indicator [Line Items]
2020	846
2019	870
2018	453
2017	310
2016	217
Prior to 2016	541
Within the Revolving Period	601
Converted to Term	208
Total	4,046
620-679 | Retail | Residential mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
2020	101
2019	97
2018	44
2017	51
2016	69
Prior to 2016	235
Within the Revolving Period	0
Converted to Term	0
Total	597
620-679 | Retail | Home equity
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	8
2018	16
2017	19
2016	12
Prior to 2016	146
Within the Revolving Period	420
Converted to Term	201
Total	822
620-679 | Retail | Automobile
Financing Receivable, Credit Quality Indicator [Line Items]
2020	523
2019	557
2018	300
2017	185
2016	102
Prior to 2016	46
Within the Revolving Period	0
Converted to Term	0
Total	1,713
620-679 | Retail | Education
Financing Receivable, Credit Quality Indicator [Line Items]
2020	27
2019	52
2018	42
2017	36
2016	31
Prior to 2016	108
Within the Revolving Period	0
Converted to Term	0
Total	296
620-679 | Retail | Other retail
Financing Receivable, Credit Quality Indicator [Line Items]
2020	195
2019	156
2018	51
2017	19
2016	3
Prior to 2016	6
Within the Revolving Period	181
Converted to Term	7
Total	618
Less than 620 | Retail
Financing Receivable, Credit Quality Indicator [Line Items]
2020	128
2019	335
2018	300
2017	266
2016	182
Prior to 2016	434
Within the Revolving Period	198
Converted to Term	222
Total	2,065
Less than 620 | Retail | Residential mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
2020	17
2019	23
2018	34
2017	56
2016	54
Prior to 2016	197
Within the Revolving Period	0
Converted to Term	0
Total	381
Less than 620 | Retail | Home equity
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1
2019	15
2018	27
2017	30
2016	18
Prior to 2016	129
Within the Revolving Period	117
Converted to Term	213
Total	550
Less than 620 | Retail | Automobile
Financing Receivable, Credit Quality Indicator [Line Items]
2020	90
2019	245
2018	202
2017	157
2016	96
Prior to 2016	48
Within the Revolving Period	0
Converted to Term	0
Total	838
Less than 620 | Retail | Education
Financing Receivable, Credit Quality Indicator [Line Items]
2020	2
2019	7
2018	13
2017	14
2016	12
Prior to 2016	56
Within the Revolving Period	0
Converted to Term	0
Total	104
Less than 620 | Retail | Other retail
Financing Receivable, Credit Quality Indicator [Line Items]
2020	18
2019	45
2018	24
2017	9
2016	2
Prior to 2016	4
Within the Revolving Period	81
Converted to Term	9
Total	192
No FICO available | Retail
Financing Receivable, Credit Quality Indicator [Line Items]
2020	47
2019	3
2018	0
2017	1
2016	0
Prior to 2016	81
Within the Revolving Period	268
Converted to Term	2
Total	402
No FICO available | Retail | Residential mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
2020	4
2019	1
2018	0
2017	1
2016	0
Prior to 2016	12
Within the Revolving Period	0
Converted to Term	0
Total	18
No FICO available | Retail | Home equity
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior to 2016	0
Within the Revolving Period	1
Converted to Term	0
Total	1
No FICO available | Retail | Automobile
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1
2019	1
2018	0
2017	0
2016	0
Prior to 2016	7
Within the Revolving Period	0
Converted to Term	0
Total	9
No FICO available | Retail | Education
Financing Receivable, Credit Quality Indicator [Line Items]
2020	6
2019	0
2018	0
2017	0
2016	0
Prior to 2016	62
Within the Revolving Period	0
Converted to Term	0
Total	68
No FICO available | Retail | Other retail
Financing Receivable, Credit Quality Indicator [Line Items]
2020	36
2019	1
2018	0
2017	0
2016	0
Prior to 2016	0
Within the Revolving Period	267
Converted to Term	2
Total	$ 306