ALLOWANCE FOR CREDIT LOSSES, NONACCRUAL LOANS AND LEASES, AND CONCENTRATIONS OF CREDIT RISK - Amortized Cost Basis by Credit Quality Indicator (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Financing Receivable, Credit Quality Indicator [Line Items]
Total	$ 128,163	$ 123,090
SBA Guaranteed PPP Loans \| Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total	787	4,200
Commercial
Financing Receivable, Credit Quality Indicator [Line Items]
2021	13,660	10,683
2020	6,289	9,445
2019	7,499	7,862
2018	4,775	3,746
2017	2,327	2,200
Prior to 2017	3,878	3,998
Within the Revolving Period	21,777	22,459
Converted to Term	145	400
Total	60,350	60,793
Commercial \| Pass
Financing Receivable, Credit Quality Indicator [Line Items]
2021	13,431	10,339
2020	6,015	8,812
2019	6,914	7,219
2018	4,184	3,419
2017	2,118	1,903
Prior to 2017	3,441	3,572
Within the Revolving Period	20,857	20,552
Converted to Term	125	340
Total	57,085	56,156
Commercial \| Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
2021	102	56
2020	128	398
2019	268	286
2018	219	180
2017	71	156
Prior to 2017	159	239
Within the Revolving Period	316	1,071
Converted to Term	1	34
Total	1,264	2,420
Commercial \| Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
2021	125	207
2020	128	199
2019	308	315
2018	350	109
2017	128	138
Prior to 2017	260	153
Within the Revolving Period	530	749
Converted to Term	17	22
Total	1,846	1,892
Commercial \| Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
2021	2	81
2020	18	36
2019	9	42
2018	22	38
2017	10	3
Prior to 2017	18	34
Within the Revolving Period	74	87
Converted to Term	2	4
Total	155	325
Commercial \| Commercial and industrial
Financing Receivable, Credit Quality Indicator [Line Items]
2021	10,363	8,226
2020	3,528	6,199
2019	3,978	4,625
2018	2,501	2,396
2017	1,527	1,301
Prior to 2017	2,144	2,311
Within the Revolving Period	20,317	18,715
Converted to Term	142	400
Total	44,500	44,173
Commercial \| Commercial and industrial \| Pass
Financing Receivable, Credit Quality Indicator [Line Items]
2021	10,218	8,036
2020	3,336	5,730
2019	3,599	4,180
2018	2,284	2,174
2017	1,426	1,157
Prior to 2017	1,863	1,980
Within the Revolving Period	19,406	17,281
Converted to Term	122	340
Total	42,254	40,878
Commercial \| Commercial and industrial \| Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
2021	47	34
2020	71	264
2019	155	163
2018	114	84
2017	41	60
Prior to 2017	64	173
Within the Revolving Period	316	771
Converted to Term	1	34
Total	809	1,583
Commercial \| Commercial and industrial \| Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
2021	97	91
2020	112	195
2019	215	248
2018	81	100
2017	50	81
Prior to 2017	201	127
Within the Revolving Period	521	600
Converted to Term	17	22
Total	1,294	1,464
Commercial \| Commercial and industrial \| Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
2021	1	65
2020	9	10
2019	9	34
2018	22	38
2017	10	3
Prior to 2017	16	31
Within the Revolving Period	74	63
Converted to Term	2	4
Total	143	248
Commercial \| Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
2021	2,839	1,999
2020	2,468	2,994
2019	3,382	3,004
2018	2,123	1,203
2017	731	713
Prior to 2017	1,258	995
Within the Revolving Period	1,460	3,744
Converted to Term	3	0
Total	14,264	14,652
Commercial \| Commercial real estate \| Pass
Financing Receivable, Credit Quality Indicator [Line Items]
2021	2,766	1,848
2020	2,417	2,836
2019	3,181	2,810
2018	1,756	1,106
2017	626	566
Prior to 2017	1,119	919
Within the Revolving Period	1,451	3,271
Converted to Term	3	0
Total	13,319	13,356
Commercial \| Commercial real estate \| Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
2021	45	19
2020	42	130
2019	113	121
2018	100	92
2017	27	94
Prior to 2017	79	48
Within the Revolving Period	0	300
Converted to Term	0	0
Total	406	804
Commercial \| Commercial real estate \| Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
2021	27	116
2020	0	2
2019	88	65
2018	267	5
2017	78	53
Prior to 2017	59	26
Within the Revolving Period	9	149
Converted to Term	0	0
Total	528	416
Commercial \| Commercial real estate \| Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
2021	1	16
2020	9	26
2019	0	8
2018	0	0
2017	0	0
Prior to 2017	1	2
Within the Revolving Period	0	24
Converted to Term	0	0
Total	11	76
Commercial \| Leases
Financing Receivable, Credit Quality Indicator [Line Items]
2021	458	458
2020	293	252
2019	139	233
2018	151	147
2017	69	186
Prior to 2017	476	692
Within the Revolving Period	0	0
Converted to Term	0	0
Total	1,586	1,968
Commercial \| Leases \| Pass
Financing Receivable, Credit Quality Indicator [Line Items]
2021	447	455
2020	262	246
2019	134	229
2018	144	139
2017	66	180
Prior to 2017	459	673
Within the Revolving Period	0	0
Converted to Term	0	0
Total	1,512	1,922
Commercial \| Leases \| Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
2021	10	3
2020	15	4
2019	0	2
2018	5	4
2017	3	2
Prior to 2017	16	18
Within the Revolving Period	0	0
Converted to Term	0	0
Total	49	33
Commercial \| Leases \| Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
2021	1	0
2020	16	2
2019	5	2
2018	2	4
2017	0	4
Prior to 2017	0	0
Within the Revolving Period	0	0
Converted to Term	0	0
Total	24	12
Commercial \| Leases \| Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
2021	0	0
2020	0	0
2019	0	0
2018	0	0
2017	0	0
Prior to 2017	1	1
Within the Revolving Period	0	0
Converted to Term	0	0
Total	1	1
Retail
Financing Receivable, Credit Quality Indicator [Line Items]
2021	19,683	17,242
2020	13,715	10,972
2019	7,020	5,335
2018	3,366	4,816
2017	3,083	4,462
Prior to 2017	7,094	6,323
Within the Revolving Period	12,688	11,755
Converted to Term	1,164	1,392
Total	67,813	62,297
Retail \| Residential mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
2021	7,700	6,512
2020	5,669	3,493
2019	2,637	1,278
2018	995	1,947
2017	1,468	2,770
Prior to 2017	4,353	3,539
Within the Revolving Period	0	0
Converted to Term	0	0
Total	22,822	19,539
Retail \| Home equity
Financing Receivable, Credit Quality Indicator [Line Items]
2021	0	6
2020	9	47
2019	43	75
2018	66	78
2017	63	50
Prior to 2017	589	833
Within the Revolving Period	10,100	9,692
Converted to Term	1,145	1,368
Total	12,015	12,149
Retail \| Automobile
Financing Receivable, Credit Quality Indicator [Line Items]
2021	7,469	4,728
2020	3,227	3,449
2019	2,065	1,864
2018	1,000	1,232
2017	561	642
Prior to 2017	227	238
Within the Revolving Period	0	0
Converted to Term	0	0
Total	14,549	12,153
Retail \| Education
Financing Receivable, Credit Quality Indicator [Line Items]
2021	3,487	4,094
2020	3,899	2,706
2019	1,798	1,606
2018	1,071	1,333
2017	890	952
Prior to 2017	1,852	1,617
Within the Revolving Period	0	0
Converted to Term	0	0
Total	12,997	12,308
Retail \| Other retail
Financing Receivable, Credit Quality Indicator [Line Items]
2021	1,027	1,902
2020	911	1,277
2019	477	512
2018	234	226
2017	101	48
Prior to 2017	73	96
Within the Revolving Period	2,588	2,063
Converted to Term	19	24
Total	5,430	6,148
Greater than 800 \| Retail
Financing Receivable, Credit Quality Indicator [Line Items]
2021	5,912	6,023
2020	5,833	4,448
2019	2,735	2,084
2018	1,170	2,306
2017	1,323	2,382
Prior to 2017	3,239	2,854
Within the Revolving Period	4,780	4,660
Converted to Term	281	344
Total	25,273	25,101
Greater than 800 \| Retail \| Residential mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
2021	2,431	2,687
2020	3,017	1,885
2019	1,230	638
2018	342	1,129
2017	672	1,615
Prior to 2017	2,139	1,755
Within the Revolving Period	0	0
Converted to Term	0	0
Total	9,831	9,709
Greater than 800 \| Retail \| Home equity
Financing Receivable, Credit Quality Indicator [Line Items]
2021	0	2
2020	2	8
2019	5	10
2018	5	7
2017	3	5
Prior to 2017	134	216
Within the Revolving Period	4,394	4,319
Converted to Term	281	344
Total	4,824	4,911
Greater than 800 \| Retail \| Automobile
Financing Receivable, Credit Quality Indicator [Line Items]
2021	1,887	1,056
2020	829	812
2019	538	424
2018	244	312
2017	148	169
Prior to 2017	57	62
Within the Revolving Period	0	0
Converted to Term	0	0
Total	3,703	2,835
Greater than 800 \| Retail \| Education
Financing Receivable, Credit Quality Indicator [Line Items]
2021	1,361	1,817
2020	1,771	1,363
2019	840	849
2018	514	781
2017	470	578
Prior to 2017	880	777
Within the Revolving Period	0	0
Converted to Term	0	0
Total	5,836	6,165
Greater than 800 \| Retail \| Other retail
Financing Receivable, Credit Quality Indicator [Line Items]
2021	233	461
2020	214	380
2019	122	163
2018	65	77
2017	30	15
Prior to 2017	29	44
Within the Revolving Period	386	341
Converted to Term	0	0
Total	1,079	1,481
740-799 \| Retail
Financing Receivable, Credit Quality Indicator [Line Items]
2021	8,311	6,864
2020	4,801	3,630
2019	2,210	1,661
2018	994	1,345
2017	836	1,190
Prior to 2017	1,806	1,597
Within the Revolving Period	4,278	3,872
Converted to Term	280	333
Total	23,516	20,492
740-799 \| Retail \| Residential mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
2021	4,015	2,931
2020	1,876	1,133
2019	746	398
2018	246	527
2017	360	743
Prior to 2017	1,086	904
Within the Revolving Period	0	0
Converted to Term	0	0
Total	8,329	6,636
740-799 \| Retail \| Home equity
Financing Receivable, Credit Quality Indicator [Line Items]
2021	0	2
2020	1	6
2019	4	7
2018	5	6
2017	7	5
Prior to 2017	122	180
Within the Revolving Period	3,514	3,234
Converted to Term	278	331
Total	3,931	3,771
740-799 \| Retail \| Automobile
Financing Receivable, Credit Quality Indicator [Line Items]
2021	2,418	1,514
2020	1,051	1,022
2019	615	531
2018	288	344
2017	156	172
Prior to 2017	58	59
Within the Revolving Period	0	0
Converted to Term	0	0
Total	4,586	3,642
740-799 \| Retail \| Education
Financing Receivable, Credit Quality Indicator [Line Items]
2021	1,555	1,797
2020	1,577	1,009
2019	672	541
2018	371	387
2017	275	251
Prior to 2017	514	423
Within the Revolving Period	0	0
Converted to Term	0	0
Total	4,964	4,408
740-799 \| Retail \| Other retail
Financing Receivable, Credit Quality Indicator [Line Items]
2021	323	620
2020	296	460
2019	173	184
2018	84	81
2017	38	19
Prior to 2017	26	31
Within the Revolving Period	764	638
Converted to Term	2	2
Total	1,706	2,035
680-739 \| Retail
Financing Receivable, Credit Quality Indicator [Line Items]
2021	3,842	3,077
2020	2,114	1,842
2019	1,193	917
2018	596	644
2017	441	541
Prior to 2017	1,041	918
Within the Revolving Period	2,447	2,193
Converted to Term	248	289
Total	11,922	10,421
680-739 \| Retail \| Residential mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
2021	1,116	784
2020	572	351
2019	335	162
2018	152	172
2017	172	295
Prior to 2017	585	458
Within the Revolving Period	0	0
Converted to Term	0	0
Total	2,932	2,222
680-739 \| Retail \| Home equity
Financing Receivable, Credit Quality Indicator [Line Items]
2021	0	1
2020	1	6
2019	7	10
2018	14	15
2017	16	8
Prior to 2017	134	179
Within the Revolving Period	1,738	1,632
Converted to Term	243	284
Total	2,153	2,135
680-739 \| Retail \| Automobile
Financing Receivable, Credit Quality Indicator [Line Items]
2021	1,968	1,347
2020	827	889
2019	500	461
2018	234	282
2017	123	138
Prior to 2017	48	47
Within the Revolving Period	0	0
Converted to Term	0	0
Total	3,700	3,164
680-739 \| Retail \| Education
Financing Receivable, Credit Quality Indicator [Line Items]
2021	512	450
2020	474	294
2019	229	173
2018	140	127
2017	107	90
Prior to 2017	262	221
Within the Revolving Period	0	0
Converted to Term	0	0
Total	1,724	1,355
680-739 \| Retail \| Other retail
Financing Receivable, Credit Quality Indicator [Line Items]
2021	246	495
2020	240	302
2019	122	111
2018	56	48
2017	23	10
Prior to 2017	12	13
Within the Revolving Period	709	561
Converted to Term	5	5
Total	1,413	1,545
620-679 \| Retail
Financing Receivable, Credit Quality Indicator [Line Items]
2021	1,339	1,040
2020	696	727
2019	517	391
2018	296	276
2017	231	192
Prior to 2017	523	491
Within the Revolving Period	662	576
Converted to Term	172	202
Total	4,436	3,895

ALLOWANCE FOR CREDIT LOSSES, NONACCRUAL LOANS AND LEASES, AND CONCENTRATIONS OF CREDIT RISK - Amortized Cost Basis by Credit Quality Indicator (Details) - USD ($)
$ in Millions