ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Amortized Cost Basis by Credit Quality Indicator (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
Total	$ 127,528	$ 119,088
Commercial
Financing Receivable, Credit Quality Indicator [Line Items]
2020	2,518
2019	11,442
2018	10,148
2017	5,559
2016	3,614
Prior to 2016	5,901
Within the Revolving Period	26,650
Converted to Term	200
Total	66,032	57,538
Commercial | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
2020	2,468
2019	11,149
2018	9,485
2017	5,216
2016	3,349
Prior to 2016	5,399
Within the Revolving Period	25,049
Converted to Term	174
Total	62,289
Commercial | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
2020	48
2019	200
2018	451
2017	171
2016	135
Prior to 2016	272
Within the Revolving Period	963
Converted to Term	4
Total	2,244
Commercial | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
2020	2
2019	66
2018	202
2017	144
2016	120
Prior to 2016	140
Within the Revolving Period	493
Converted to Term	18
Total	1,185
Commercial | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	27
2018	10
2017	28
2016	10
Prior to 2016	90
Within the Revolving Period	145
Converted to Term	4
Total	314
Commercial | Commercial
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1,482
2019	7,670
2018	5,867
2017	3,326
2016	2,085
Prior to 2016	3,403
Within the Revolving Period	25,059
Converted to Term	200
Total	49,092	41,479
Commercial | Commercial | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1,480
2019	7,457
2018	5,444
2017	3,079
2016	1,901
Prior to 2016	3,035
Within the Revolving Period	23,655
Converted to Term	174
Total	46,225
Commercial | Commercial | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	126
2018	264
2017	88
2016	75
Prior to 2016	177
Within the Revolving Period	766
Converted to Term	4
Total	1,500
Commercial | Commercial | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
2020	2
2019	60
2018	150
2017	136
2016	99
Prior to 2016	104
Within the Revolving Period	493
Converted to Term	18
Total	1,062
Commercial | Commercial | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	27
2018	9
2017	23
2016	10
Prior to 2016	87
Within the Revolving Period	145
Converted to Term	4
Total	305
Commercial | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
2020	946
2019	3,394
2018	3,951
2017	2,022
2016	1,243
Prior to 2016	1,355
Within the Revolving Period	1,591
Converted to Term	0
Total	14,502	13,522
Commercial | Commercial real estate | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
2020	898
2019	3,322
2018	3,750
2017	1,937
2016	1,187
Prior to 2016	1,257
Within the Revolving Period	1,394
Converted to Term	0
Total	13,745
Commercial | Commercial real estate | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
2020	48
2019	72
2018	181
2017	78
2016	56
Prior to 2016	93
Within the Revolving Period	197
Converted to Term	0
Total	725
Commercial | Commercial real estate | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	19
2017	2
2016	0
Prior to 2016	3
Within the Revolving Period	0
Converted to Term	0
Total	24
Commercial | Commercial real estate | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	1
2017	5
2016	0
Prior to 2016	2
Within the Revolving Period	0
Converted to Term	0
Total	8
Commercial | Leases
Financing Receivable, Credit Quality Indicator [Line Items]
2020	90
2019	378
2018	330
2017	211
2016	286
Prior to 2016	1,143
Within the Revolving Period	0
Converted to Term	0
Total	2,438	2,537
Commercial | Leases | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
2020	90
2019	370
2018	291
2017	200
2016	261
Prior to 2016	1,107
Within the Revolving Period	0
Converted to Term	0
Total	2,319
Commercial | Leases | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	2
2018	6
2017	5
2016	4
Prior to 2016	2
Within the Revolving Period	0
Converted to Term	0
Total	19
Commercial | Leases | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	6
2018	33
2017	6
2016	21
Prior to 2016	33
Within the Revolving Period	0
Converted to Term	0
Total	99
Commercial | Leases | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior to 2016	1
Within the Revolving Period	0
Converted to Term	0
Total	1
Retail
Financing Receivable, Credit Quality Indicator [Line Items]
2020	3,694
2019	14,652
2018	7,608
2017	6,794
2016	6,073
Prior to 2016	8,680
Within the Revolving Period	12,701
Converted to Term	1,294
Total	61,496	61,550
Retail | Residential mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1,172
2019	4,527
2018	1,956
2017	2,765
2016	3,508
Prior to 2016	4,793
Within the Revolving Period	0
Converted to Term	0
Total	18,721	19,083
Retail | Home equity
Financing Receivable, Credit Quality Indicator [Line Items]
2020	2
2019	24
2018	47
2017	73
2016	46
Prior to 2016	1,077
Within the Revolving Period	10,429
Converted to Term	1,294
Total	12,992	13,154
Retail | Automobile
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1,269
2019	4,545
2018	2,623
2017	1,886
2016	1,149
Prior to 2016	685
Within the Revolving Period	0
Converted to Term	0
Total	12,157	12,120
Retail | Education
Financing Receivable, Credit Quality Indicator [Line Items]
2020	625
2019	3,174
2018	2,086
2017	1,722
2016	1,289
Prior to 2016	1,991
Within the Revolving Period	0
Converted to Term	0
Total	10,887	10,347
Retail | Other retail
Financing Receivable, Credit Quality Indicator [Line Items]
2020	626
2019	2,382
2018	896
2017	348
2016	81
Prior to 2016	134
Within the Revolving Period	2,272
Converted to Term	0
Total	6,739	$ 6,846
Greater than 800 | Retail
Financing Receivable, Credit Quality Indicator [Line Items]
2020	990
2019	4,372
2018	2,620
2017	2,934
2016	2,986
Prior to 2016	3,828
Within the Revolving Period	4,614
Converted to Term	339
Total	22,683
Greater than 800 | Retail | Residential mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
2020	328
2019	1,694
2018	950
2017	1,541
2016	2,001
Prior to 2016	2,459
Within the Revolving Period	0
Converted to Term	0
Total	8,973
Greater than 800 | Retail | Home equity
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1
2019	10
2018	15
2017	9
2016	6
Prior to 2016	288
Within the Revolving Period	4,314
Converted to Term	339
Total	4,982
Greater than 800 | Retail | Automobile
Financing Receivable, Credit Quality Indicator [Line Items]
2020	302
2019	935
2018	541
2017	441
2016	289
Prior to 2016	175
Within the Revolving Period	0
Converted to Term	0
Total	2,683
Greater than 800 | Retail | Education
Financing Receivable, Credit Quality Indicator [Line Items]
2020	221
2019	1,159
2018	880
2017	853
2016	669
Prior to 2016	848
Within the Revolving Period	0
Converted to Term	0
Total	4,630
Greater than 800 | Retail | Other retail
Financing Receivable, Credit Quality Indicator [Line Items]
2020	138
2019	574
2018	234
2017	90
2016	21
Prior to 2016	58
Within the Revolving Period	300
Converted to Term	0
Total	1,415
740-799 | Retail
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1,545
2019	5,550
2018	2,504
2017	2,061
2016	1,762
Prior to 2016	2,235
Within the Revolving Period	4,087
Converted to Term	302
Total	20,046
740-799 | Retail | Residential mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
2020	634
2019	2,026
2018	647
2017	828
2016	1,017
Prior to 2016	1,220
Within the Revolving Period	0
Converted to Term	0
Total	6,372
740-799 | Retail | Home equity
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1
2019	7
2018	8
2017	7
2016	4
Prior to 2016	238
Within the Revolving Period	3,423
Converted to Term	302
Total	3,990
740-799 | Retail | Automobile
Financing Receivable, Credit Quality Indicator [Line Items]
2020	393
2019	1,304
2018	738
2017	529
2016	312
Prior to 2016	174
Within the Revolving Period	0
Converted to Term	0
Total	3,450
740-799 | Retail | Education
Financing Receivable, Credit Quality Indicator [Line Items]
2020	294
2019	1,364
2018	793
2017	567
2016	397
Prior to 2016	562
Within the Revolving Period	0
Converted to Term	0
Total	3,977
740-799 | Retail | Other retail
Financing Receivable, Credit Quality Indicator [Line Items]
2020	223
2019	849
2018	318
2017	130
2016	32
Prior to 2016	41
Within the Revolving Period	664
Converted to Term	0
Total	2,257
680-739 | Retail
Financing Receivable, Credit Quality Indicator [Line Items]
2020	814
2019	3,085
2018	1,501
2017	1,050
2016	800
Prior to 2016	1,293
Within the Revolving Period	2,563
Converted to Term	272
Total	11,378
680-739 | Retail | Residential mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
2020	192
2019	687
2018	280
2017	294
2016	367
Prior to 2016	614
Within the Revolving Period	0
Converted to Term	0
Total	2,434
680-739 | Retail | Home equity
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	3
2018	8
2017	9
2016	6
Prior to 2016	224
Within the Revolving Period	1,915
Converted to Term	272
Total	2,437
680-739 | Retail | Automobile
Financing Receivable, Credit Quality Indicator [Line Items]
2020	355
2019	1,233
2018	676
2017	441
2016	252
Prior to 2016	135
Within the Revolving Period	0
Converted to Term	0
Total	3,092
680-739 | Retail | Education
Financing Receivable, Credit Quality Indicator [Line Items]
2020	105
2019	553
2018	321
2017	222
2016	156
Prior to 2016	298
Within the Revolving Period	0
Converted to Term	0
Total	1,655
680-739 | Retail | Other retail
Financing Receivable, Credit Quality Indicator [Line Items]
2020	162
2019	609
2018	216
2017	84
2016	19
Prior to 2016	22
Within the Revolving Period	648
Converted to Term	0
Total	1,760
620-679 | Retail
Financing Receivable, Credit Quality Indicator [Line Items]
2020	286
2019	1,219
2018	594
2017	402
2016	284
Prior to 2016	619
Within the Revolving Period	799
Converted to Term	174
Total	4,377
620-679 | Retail | Residential mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
2020	12
2019	102
2018	44
2017	56
2016	79
Prior to 2016	237
Within the Revolving Period	0
Converted to Term	0
Total	530
620-679 | Retail | Home equity
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	1
2018	6
2017	15
2016	9
Prior to 2016	160
Within the Revolving Period	570
Converted to Term	174
Total	935
620-679 | Retail | Automobile
Financing Receivable, Credit Quality Indicator [Line Items]
2020	191
2019	758
2018	391
2017	248
2016	145
Prior to 2016	84
Within the Revolving Period	0
Converted to Term	0
Total	1,817
620-679 | Retail | Education
Financing Receivable, Credit Quality Indicator [Line Items]
2020	5
2019	86
2018	69
2017	55
2016	45
Prior to 2016	130
Within the Revolving Period	0
Converted to Term	0
Total	390
620-679 | Retail | Other retail
Financing Receivable, Credit Quality Indicator [Line Items]
2020	78
2019	272
2018	84
2017	28
2016	6
Prior to 2016	8
Within the Revolving Period	229
Converted to Term	0
Total	705
Less than 620 | Retail
Financing Receivable, Credit Quality Indicator [Line Items]
2020	36
2019	420
2018	389
2017	347
2016	241
Prior to 2016	607
Within the Revolving Period	324
Converted to Term	207
Total	2,571
Less than 620 | Retail | Residential mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
2020	4
2019	16
2018	35
2017	46
2016	44
Prior to 2016	251
Within the Revolving Period	0
Converted to Term	0
Total	396
Less than 620 | Retail | Home equity
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	3
2018	10
2017	33
2016	21
Prior to 2016	166
Within the Revolving Period	207
Converted to Term	207
Total	647
Less than 620 | Retail | Automobile
Financing Receivable, Credit Quality Indicator [Line Items]
2020	26
2019	314
2018	277
2017	227
2016	151
Prior to 2016	99
Within the Revolving Period	0
Converted to Term	0
Total	1,094
Less than 620 | Retail | Education
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	12
2018	23
2017	25
2016	22
Prior to 2016	86
Within the Revolving Period	0
Converted to Term	0
Total	168
Less than 620 | Retail | Other retail
Financing Receivable, Credit Quality Indicator [Line Items]
2020	6
2019	75
2018	44
2017	16
2016	3
Prior to 2016	5
Within the Revolving Period	117
Converted to Term	0
Total	266
No FICO available | Retail
Financing Receivable, Credit Quality Indicator [Line Items]
2020	23
2019	6
2018	0
2017	0
2016	0
Prior to 2016	98
Within the Revolving Period	314
Converted to Term	0
Total	441
No FICO available | Retail | Residential mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
2020	2
2019	2
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	1
Within the Revolving Period	0
Converted to Term	0
Total	1
No FICO available | Retail | Automobile
Financing Receivable, Credit Quality Indicator [Line Items]
2020	2
2019	1
2018	0
2017	0
2016	0
Prior to 2016	18
Within the Revolving Period	0
Converted to Term	0
Total	21
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	67
Within the Revolving Period	0
Converted to Term	0
Total	67
No FICO available | Retail | Other retail
Financing Receivable, Credit Quality Indicator [Line Items]
2020	19
2019	3
2018	0
2017	0
2016	0
Prior to 2016	0
Within the Revolving Period	314
Converted to Term	0
Total	$ 336