ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Amortized Cost Basis by Credit Quality Indicator (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
Total	$ 125,713	$ 119,088
Commercial
Financing Receivable, Credit Quality Indicator [Line Items]
2020	8,365
2019	11,387
2018	9,842
2017	5,325
2016	3,229
Prior to 2016	5,426
Within the Revolving Period	20,918
Converted to Term	438
Total	64,930	57,538
Commercial | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
2020	8,075
2019	10,853
2018	9,003
2017	4,873
2016	2,837
Prior to 2016	4,883
Within the Revolving Period	18,707
Converted to Term	393
Total	59,624
Commercial | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
2020	190
2019	317
2018	437
2017	269
2016	194
Prior to 2016	292
Within the Revolving Period	1,187
Converted to Term	22
Total	2,908
Commercial | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
2020	92
2019	149
2018	347
2017	128
2016	179
Prior to 2016	123
Within the Revolving Period	851
Converted to Term	19
Total	1,888
Commercial | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
2020	8
2019	68
2018	55
2017	55
2016	19
Prior to 2016	128
Within the Revolving Period	173
Converted to Term	4
Total	510
Commercial | Commercial
Financing Receivable, Credit Quality Indicator [Line Items]
2020	6,844
2019	7,487
2018	5,588
2017	3,197
2016	1,814
Prior to 2016	3,149
Within the Revolving Period	19,500
Converted to Term	438
Total	48,017	41,479
Commercial | Commercial | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
2020	6,585
2019	7,127
2018	5,054
2017	2,897
2016	1,564
Prior to 2016	2,724
Within the Revolving Period	17,515
Converted to Term	393
Total	43,859
Commercial | Commercial | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
2020	159
2019	245
2018	287
2017	131
2016	126
Prior to 2016	209
Within the Revolving Period	1,110
Converted to Term	22
Total	2,289
Commercial | Commercial | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
2020	92
2019	89
2018	238
2017	120
2016	119
Prior to 2016	119
Within the Revolving Period	702
Converted to Term	19
Total	1,498
Commercial | Commercial | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
2020	8
2019	26
2018	9
2017	49
2016	5
Prior to 2016	97
Within the Revolving Period	173
Converted to Term	4
Total	371
Commercial | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1,258
2019	3,541
2018	3,941
2017	1,931
2016	1,157
Prior to 2016	1,239
Within the Revolving Period	1,418
Converted to Term	0
Total	14,485	13,522
Commercial | Commercial real estate | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1,227
2019	3,375
2018	3,674
2017	1,792
2016	1,037
Prior to 2016	1,152
Within the Revolving Period	1,192
Converted to Term	0
Total	13,449
Commercial | Commercial real estate | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
2020	31
2019	70
2018	145
2017	132
2016	64
Prior to 2016	82
Within the Revolving Period	77
Converted to Term	0
Total	601
Commercial | Commercial real estate | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	58
2018	106
2017	2
2016	56
Prior to 2016	3
Within the Revolving Period	149
Converted to Term	0
Total	374
Commercial | Commercial real estate | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	38
2018	16
2017	5
2016	0
Prior to 2016	2
Within the Revolving Period	0
Converted to Term	0
Total	61
Commercial | Leases
Financing Receivable, Credit Quality Indicator [Line Items]
2020	263
2019	359
2018	313
2017	197
2016	258
Prior to 2016	1,038
Within the Revolving Period	0
Converted to Term	0
Total	2,428	2,537
Commercial | Leases | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
2020	263
2019	351
2018	275
2017	184
2016	236
Prior to 2016	1,007
Within the Revolving Period	0
Converted to Term	0
Total	2,316
Commercial | Leases | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	2
2018	5
2017	6
2016	4
Prior to 2016	1
Within the Revolving Period	0
Converted to Term	0
Total	18
Commercial | Leases | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	2
2018	3
2017	6
2016	4
Prior to 2016	1
Within the Revolving Period	0
Converted to Term	0
Total	16
Commercial | Leases | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	4
2018	30
2017	1
2016	14
Prior to 2016	29
Within the Revolving Period	0
Converted to Term	0
Total	78
Commercial | SBA Guaranteed PPP Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total	4,700
Retail
Financing Receivable, Credit Quality Indicator [Line Items]
2020	8,221
2019	13,288
2018	6,582
2017	5,963
2016	5,484
Prior to 2016	7,857
Within the Revolving Period	11,903
Converted to Term	1,485
Total	60,783	61,550
Retail | Residential mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
2020	3,078
2019	4,198
2018	1,714
2017	2,497
2016	3,317
Prior to 2016	4,441
Within the Revolving Period	0
Converted to Term	0
Total	19,245	19,083
Retail | Home equity
Financing Receivable, Credit Quality Indicator [Line Items]
2020	3
2019	27
2018	48
2017	79
2016	49
Prior to 2016	1,001
Within the Revolving Period	9,876
Converted to Term	1,458
Total	12,541	13,154
Retail | Automobile
Financing Receivable, Credit Quality Indicator [Line Items]
2020	2,343
2019	4,182
2018	2,373
2017	1,663
2016	969
Prior to 2016	498
Within the Revolving Period	0
Converted to Term	0
Total	12,028	12,120
Retail | Education
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1,759
2019	2,856
2018	1,692
2017	1,411
2016	1,079
Prior to 2016	1,794
Within the Revolving Period	0
Converted to Term	0
Total	10,591	10,347
Retail | Other retail
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1,038
2019	2,025
2018	755
2017	313
2016	70
Prior to 2016	123
Within the Revolving Period	2,027
Converted to Term	27
Total	6,378	$ 6,846
Greater than 800 | Retail
Financing Receivable, Credit Quality Indicator [Line Items]
2020	2,509
2019	4,651
2018	2,393
2017	2,747
2016	2,868
Prior to 2016	3,618
Within the Revolving Period	4,624
Converted to Term	379
Total	23,789
Greater than 800 | Retail | Residential mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1,067
2019	2,018
2018	876
2017	1,488
2016	1,986
Prior to 2016	2,353
Within the Revolving Period	0
Converted to Term	0
Total	9,788
Greater than 800 | Retail | Home equity
Financing Receivable, Credit Quality Indicator [Line Items]
2020	2
2019	10
2018	14
2017	9
2016	6
Prior to 2016	269
Within the Revolving Period	4,331
Converted to Term	379
Total	5,020
Greater than 800 | Retail | Automobile
Financing Receivable, Credit Quality Indicator [Line Items]
2020	550
2019	924
2018	521
2017	412
2016	255
Prior to 2016	131
Within the Revolving Period	0
Converted to Term	0
Total	2,793
Greater than 800 | Retail | Education
Financing Receivable, Credit Quality Indicator [Line Items]
2020	651
2019	1,165
2018	773
2017	751
2016	601
Prior to 2016	811
Within the Revolving Period	0
Converted to Term	0
Total	4,752
Greater than 800 | Retail | Other retail
Financing Receivable, Credit Quality Indicator [Line Items]
2020	239
2019	534
2018	209
2017	87
2016	20
Prior to 2016	54
Within the Revolving Period	293
Converted to Term	0
Total	1,436
740-799 | Retail
Financing Receivable, Credit Quality Indicator [Line Items]
2020	3,474
2019	4,744
2018	2,101
2017	1,707
2016	1,466
Prior to 2016	1,913
Within the Revolving Period	3,777
Converted to Term	347
Total	19,529
740-799 | Retail | Residential mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1,574
2019	1,594
2018	520
2017	677
2016	858
Prior to 2016	1,059
Within the Revolving Period	0
Converted to Term	0
Total	6,282
740-799 | Retail | Home equity
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1
2019	7
2018	7
2017	5
2016	5
Prior to 2016	216
Within the Revolving Period	3,164
Converted to Term	345
Total	3,750
740-799 | Retail | Automobile
Financing Receivable, Credit Quality Indicator [Line Items]
2020	740
2019	1,218
2018	679
2017	466
2016	261
Prior to 2016	124
Within the Revolving Period	0
Converted to Term	0
Total	3,488
740-799 | Retail | Education
Financing Receivable, Credit Quality Indicator [Line Items]
2020	820
2019	1,199
2018	627
2017	443
2016	314
Prior to 2016	475
Within the Revolving Period	0
Converted to Term	0
Total	3,878
740-799 | Retail | Other retail
Financing Receivable, Credit Quality Indicator [Line Items]
2020	339
2019	726
2018	268
2017	116
2016	28
Prior to 2016	39
Within the Revolving Period	613
Converted to Term	2
Total	2,131
680-739 | Retail
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1,587
2019	2,512
2018	1,257
2017	859
2016	678
Prior to 2016	1,143
Within the Revolving Period	2,309
Converted to Term	310
Total	10,655
680-739 | Retail | Residential mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
2020	387
2019	459
2018	242
2017	226
2016	329
Prior to 2016	556
Within the Revolving Period	0
Converted to Term	0
Total	2,199
680-739 | Retail | Home equity
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	3
2018	6
2017	12
2016	7
Prior to 2016	213
Within the Revolving Period	1,730
Converted to Term	304
Total	2,275
680-739 | Retail | Automobile
Financing Receivable, Credit Quality Indicator [Line Items]
2020	667
2019	1,126
2018	600
2017	384
2016	209
Prior to 2016	98
Within the Revolving Period	0
Converted to Term	0
Total	3,084
680-739 | Retail | Education
Financing Receivable, Credit Quality Indicator [Line Items]
2020	267
2019	424
2018	231
2017	163
2016	117
Prior to 2016	258
Within the Revolving Period	0
Converted to Term	0
Total	1,460
680-739 | Retail | Other retail
Financing Receivable, Credit Quality Indicator [Line Items]
2020	266
2019	500
2018	178
2017	74
2016	16
Prior to 2016	18
Within the Revolving Period	579
Converted to Term	6
Total	1,637
620-679 | Retail
Financing Receivable, Credit Quality Indicator [Line Items]
2020	540
2019	1,017
2018	504
2017	355
2016	261
Prior to 2016	581
Within the Revolving Period	676
Converted to Term	210
Total	4,144
620-679 | Retail | Residential mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
2020	46
2019	105
2018	40
2017	58
2016	89
Prior to 2016	240
Within the Revolving Period	0
Converted to Term	0
Total	578
620-679 | Retail | Home equity
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	3
2018	10
2017	20
2016	11
Prior to 2016	150
Within the Revolving Period	480
Converted to Term	203
Total	877
620-679 | Retail | Automobile
Financing Receivable, Credit Quality Indicator [Line Items]
2020	332
2019	646
2018	342
2017	215
2016	123
Prior to 2016	64
Within the Revolving Period	0
Converted to Term	0
Total	1,722
620-679 | Retail | Education
Financing Receivable, Credit Quality Indicator [Line Items]
2020	20
2019	59
2018	46
2017	38
2016	34
Prior to 2016	119
Within the Revolving Period	0
Converted to Term	0
Total	316
620-679 | Retail | Other retail
Financing Receivable, Credit Quality Indicator [Line Items]
2020	142
2019	204
2018	66
2017	24
2016	4
Prior to 2016	8
Within the Revolving Period	196
Converted to Term	7
Total	651
Less than 620 | Retail
Financing Receivable, Credit Quality Indicator [Line Items]
2020	67
2019	362
2018	327
2017	295
2016	211
Prior to 2016	514
Within the Revolving Period	265
Converted to Term	237
Total	2,278
Less than 620 | Retail | Residential mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1
2019	21
2018	36
2017	48
2016	55
Prior to 2016	221
Within the Revolving Period	0
Converted to Term	0
Total	382
Less than 620 | Retail | Home equity
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	4
2018	11
2017	33
2016	20
Prior to 2016	153
Within the Revolving Period	170
Converted to Term	227
Total	618
Less than 620 | Retail | Automobile
Financing Receivable, Credit Quality Indicator [Line Items]
2020	52
2019	268
2018	231
2017	186
2016	121
Prior to 2016	69
Within the Revolving Period	0
Converted to Term	0
Total	927
Less than 620 | Retail | Education
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1
2019	9
2018	15
2017	16
2016	13
Prior to 2016	67
Within the Revolving Period	0
Converted to Term	0
Total	121
Less than 620 | Retail | Other retail
Financing Receivable, Credit Quality Indicator [Line Items]
2020	13
2019	60
2018	34
2017	12
2016	2
Prior to 2016	4
Within the Revolving Period	95
Converted to Term	10
Total	230
No FICO available | Retail
Financing Receivable, Credit Quality Indicator [Line Items]
2020	44
2019	2
2018	0
2017	0
2016	0
Prior to 2016	88
Within the Revolving Period	252
Converted to Term	2
Total	388
No FICO available | Retail | Residential mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
2020	3
2019	1
2018	0
2017	0
2016	0
Prior to 2016	12
Within the Revolving Period	0
Converted to Term	0
Total	16
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	2
2019	0
2018	0
2017	0
2016	0
Prior to 2016	12
Within the Revolving Period	0
Converted to Term	0
Total	14
No FICO available | Retail | Education
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior to 2016	64
Within the Revolving Period	0
Converted to Term	0
Total	64
No FICO available | Retail | Other retail
Financing Receivable, Credit Quality Indicator [Line Items]
2020	39
2019	1
2018	0
2017	0
2016	0
Prior to 2016	0
Within the Revolving Period	251
Converted to Term	2
Total	$ 293