ALLOWANCE FOR CREDIT LOSSES, NONACCRUAL LOANS AND LEASES, AND CONCENTRATIONS OF CREDIT RISK - Amortized Cost Basis by Credit Quality Indicator (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Financing Receivable, Credit Quality Indicator [Line Items]
Total	$ 131,305	$ 128,163
Commercial
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	2,804	13,660
Year two	13,126	6,289
Year three	5,930	7,499
Year four	6,573	4,775
Year five	4,234	2,327
Prior to year five	5,612	3,878
Within the Revolving Period	23,100	21,777
Converted to Term	142	145
Total	61,521	60,350
Commercial | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	2,796	13,431
Year two	12,892	6,015
Year three	5,702	6,914
Year four	5,923	4,184
Year five	3,700	2,118
Prior to year five	5,055	3,441
Within the Revolving Period	22,187	20,857
Converted to Term	121	125
Total	58,376	57,085
Commercial | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	1	102
Year two	130	128
Year three	108	268
Year four	335	219
Year five	201	71
Prior to year five	194	159
Within the Revolving Period	395	316
Converted to Term	1	1
Total	1,365	1,264
Commercial | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	0	125
Year two	91	128
Year three	92	308
Year four	311	350
Year five	323	128
Prior to year five	336	260
Within the Revolving Period	422	530
Converted to Term	17	17
Total	1,592	1,846
Commercial | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	7	2
Year two	13	18
Year three	28	9
Year four	4	22
Year five	10	10
Prior to year five	27	18
Within the Revolving Period	96	74
Converted to Term	3	2
Total	188	155
Commercial | Commercial and industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	1,952	10,363
Year two	9,994	3,528
Year three	3,173	3,978
Year four	3,241	2,501
Year five	2,287	1,527
Prior to year five	3,309	2,144
Within the Revolving Period	21,629	20,317
Converted to Term	139	142
Total	45,724	44,500
Commercial | Commercial and industrial | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	1,944	10,218
Year two	9,819	3,336
Year three	3,007	3,599
Year four	2,908	2,284
Year five	2,094	1,426
Prior to year five	2,975	1,863
Within the Revolving Period	20,729	19,406
Converted to Term	118	122
Total	43,594	42,254
Commercial | Commercial and industrial | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	1	47
Year two	74	71
Year three	58	155
Year four	107	114
Year five	105	41
Prior to year five	114	64
Within the Revolving Period	385	316
Converted to Term	1	1
Total	845	809
Commercial | Commercial and industrial | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	0	97
Year two	89	112
Year three	89	215
Year four	222	81
Year five	78	50
Prior to year five	195	201
Within the Revolving Period	419	521
Converted to Term	17	17
Total	1,109	1,294
Commercial | Commercial and industrial | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	7	1
Year two	12	9
Year three	19	9
Year four	4	22
Year five	10	10
Prior to year five	25	16
Within the Revolving Period	96	74
Converted to Term	3	2
Total	176	143
Commercial | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	784	2,839
Year two	2,723	2,468
Year three	2,477	3,382
Year four	3,201	2,123
Year five	1,801	731
Prior to year five	1,808	1,258
Within the Revolving Period	1,471	1,460
Converted to Term	3	3
Total	14,268	14,264
Commercial | Commercial real estate | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	784	2,766
Year two	2,667	2,417
Year three	2,420	3,181
Year four	2,889	1,756
Year five	1,465	626
Prior to year five	1,587	1,119
Within the Revolving Period	1,458	1,451
Converted to Term	3	3
Total	13,273	13,319
Commercial | Commercial real estate | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	0	45
Year two	54	42
Year three	48	113
Year four	228	100
Year five	93	27
Prior to year five	79	79
Within the Revolving Period	10	0
Converted to Term	0	0
Total	512	406
Commercial | Commercial real estate | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	0	27
Year two	1	0
Year three	0	88
Year four	84	267
Year five	243	78
Prior to year five	141	59
Within the Revolving Period	3	9
Converted to Term	0	0
Total	472	528
Commercial | Commercial real estate | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	0	1
Year two	1	9
Year three	9	0
Year four	0	0
Year five	0	0
Prior to year five	1	1
Within the Revolving Period	0	0
Converted to Term	0	0
Total	11	11
Commercial | Leases
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	68	458
Year two	409	293
Year three	280	139
Year four	131	151
Year five	146	69
Prior to year five	495	476
Within the Revolving Period	0	0
Converted to Term	0	0
Total	1,529	1,586
Commercial | Leases | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	68	447
Year two	406	262
Year three	275	134
Year four	126	144
Year five	141	66
Prior to year five	493	459
Within the Revolving Period	0	0
Converted to Term	0	0
Total	1,509	1,512
Commercial | Leases | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	0	10
Year two	2	15
Year three	2	0
Year four	0	5
Year five	3	3
Prior to year five	1	16
Within the Revolving Period	0	0
Converted to Term	0	0
Total	8	49
Commercial | Leases | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	0	1
Year two	1	16
Year three	3	5
Year four	5	2
Year five	2	0
Prior to year five	0	0
Within the Revolving Period	0	0
Converted to Term	0	0
Total	11	24
Commercial | Leases | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
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	0	0
Converted to Term	0	0
Total	1	1
Retail
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	3,898	19,683
Year two	19,483	13,715
Year three	12,716	7,020
Year four	6,344	3,366
Year five	3,023	3,083
Prior to year five	9,847	7,094
Within the Revolving Period	13,239	12,688
Converted to Term	1,234	1,164
Total	69,784	67,813
Retail | Residential mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	1,427	7,700
Year two	7,900	5,669
Year three	5,495	2,637
Year four	2,477	995
Year five	963	1,468
Prior to year five	5,949	4,353
Within the Revolving Period	0	0
Converted to Term	0	0
Total	24,211	22,822
Retail | Home equity
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	0	0
Year two	1	9
Year three	7	43
Year four	41	66
Year five	63	63
Prior to year five	593	589
Within the Revolving Period	10,343	10,100
Converted to Term	1,216	1,145
Total	12,264	12,015
Retail | Automobile
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	1,536	7,469
Year two	6,827	3,227
Year three	2,862	2,065
Year four	1,789	1,000
Year five	838	561
Prior to year five	587	227
Within the Revolving Period	0	0
Converted to Term	0	0
Total	14,439	14,549
Retail | Education
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	698	3,487
Year two	3,837	3,899
Year three	3,581	1,798
Year four	1,641	1,071
Year five	972	890
Prior to year five	2,577	1,852
Within the Revolving Period	0	0
Converted to Term	0	0
Total	13,306	12,997
Retail | Other retail
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	237	1,027
Year two	918	911
Year three	771	477
Year four	396	234
Year five	187	101
Prior to year five	141	73
Within the Revolving Period	2,896	2,588
Converted to Term	18	19
Total	5,564	5,430
Greater than 800 | Retail
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	975	5,912
Year two	6,865	5,833
Year three	5,718	2,735
Year four	2,563	1,170
Year five	1,068	1,323
Prior to year five	4,456	3,239
Within the Revolving Period	4,906	4,780
Converted to Term	310	281
Total	26,861	25,273
Greater than 800 | Retail | Residential mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	351	2,431
Year two	3,332	3,017
Year three	3,066	1,230
Year four	1,184	342
Year five	322	672
Prior to year five	2,868	2,139
Within the Revolving Period	0	0
Converted to Term	0	0
Total	11,123	9,831
Greater than 800 | Retail | Home equity
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	0	0
Year two	1	2
Year three	1	5
Year four	4	5
Year five	5	3
Prior to year five	122	134
Within the Revolving Period	4,492	4,394
Converted to Term	310	281
Total	4,935	4,824
Greater than 800 | Retail | Automobile
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	369	1,887
Year two	1,662	829
Year three	780	538
Year four	481	244
Year five	210	148
Prior to year five	154	57
Within the Revolving Period	0	0
Converted to Term	0	0
Total	3,656	3,703
Greater than 800 | Retail | Education
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	213	1,361
Year two	1,641	1,771
Year three	1,687	840
Year four	792	514
Year five	479	470
Prior to year five	1,261	880
Within the Revolving Period	0	0
Converted to Term	0	0
Total	6,073	5,836
Greater than 800 | Retail | Other retail
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	42	233
Year two	229	214
Year three	184	122
Year four	102	65
Year five	52	30
Prior to year five	51	29
Within the Revolving Period	414	386
Converted to Term	0	0
Total	1,074	1,079
740-799 | Retail
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	1,726	8,311
Year two	7,520	4,801
Year three	4,242	2,210
Year four	1,935	994
Year five	875	836
Prior to year five	2,484	1,806
Within the Revolving Period	4,377	4,278
Converted to Term	298	280
Total	23,457	23,516
740-799 | Retail | Residential mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	866	4,015
Year two	3,423	1,876
Year three	1,690	746
Year four	673	246
Year five	240	360
Prior to year five	1,429	1,086
Within the Revolving Period	0	0
Converted to Term	0	0
Total	8,321	8,329
740-799 | Retail | Home equity
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	0	0
Year two	0	1
Year three	1	4
Year four	4	5
Year five	6	7
Prior to year five	110	122
Within the Revolving Period	3,564	3,514
Converted to Term	296	278
Total	3,981	3,931
740-799 | Retail | Automobile
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	487	2,418
Year two	2,203	1,051
Year three	915	615
Year four	531	288
Year five	238	156
Prior to year five	157	58
Within the Revolving Period	0	0
Converted to Term	0	0
Total	4,531	4,586
740-799 | Retail | Education
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	311	1,555
Year two	1,600	1,577
Year three	1,391	672
Year four	587	371
Year five	325	275
Prior to year five	739	514
Within the Revolving Period	0	0
Converted to Term	0	0
Total	4,953	4,964
740-799 | Retail | Other retail
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	62	323
Year two	294	296
Year three	245	173
Year four	140	84
Year five	66	38
Prior to year five	49	26
Within the Revolving Period	813	764
Converted to Term	2	2
Total	1,671	1,706
680-739 | Retail
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	830	3,842
Year two	3,457	2,114
Year three	1,869	1,193
Year four	1,009	596
Year five	519	441
Prior to year five	1,446	1,041
Within the Revolving Period	2,584	2,447
Converted to Term	262	248
Total	11,976	11,922
680-739 | Retail | Residential mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	192	1,116
Year two	968	572
Year three	540	335
Year four	275	152
Year five	145	172
Prior to year five	800	585
Within the Revolving Period	0	0
Converted to Term	0	0
Total	2,920	2,932
680-739 | Retail | Home equity
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	0	0
Year two	0	1
Year three	1	7
Year four	8	14
Year five	13	16
Prior to year five	140	134
Within the Revolving Period	1,774	1,738
Converted to Term	258	243
Total	2,194	2,153
680-739 | Retail | Automobile
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	423	1,968
Year two	1,751	827
Year three	701	500
Year four	414	234
Year five	190	123
Prior to year five	124	48
Within the Revolving Period	0	0
Converted to Term	0	0
Total	3,603	3,700
680-739 | Retail | Education
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	161	512
Year two	513	474
Year three	425	229
Year four	209	140
Year five	126	107
Prior to year five	354	262
Within the Revolving Period	0	0
Converted to Term	0	0
Total	1,788	1,724
680-739 | Retail | Other retail
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	54	246
Year two	225	240
Year three	202	122
Year four	103	56
Year five	45	23
Prior to year five	28	12
Within the Revolving Period	810	709
Converted to Term	4	5
Total	1,471	1,413
620-679 | Retail
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	288	1,339
Year two	1,248	696
Year three	596	517
Year four	474	296
Year five	267	231
Prior to year five	724	523
Within the Revolving Period	748	662
Converted to Term	173	172
Total	4,518	4,436
620-679 | Retail | Residential mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	18	111
Year two	127	130
Year three	114	161
Year four	170	93
Year five	93	107
Prior to year five	399	276
Within the Revolving Period	0	0
Converted to Term	0	0
Total	921	878
620-679 | Retail | Home equity
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	0	0
Year two	0	3
Year three	2	11
Year four	9	19
Year five	19	17
Prior to year five	118	112
Within the Revolving Period	399	363
Converted to Term	168	167
Total	715	692
620-679 | Retail | Automobile
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	223	1,029
Year two	919	378
Year three	317	257
Year four	218	131
Year five	109	72
Prior to year five	77	32
Within the Revolving Period	0	0
Converted to Term	0	0
Total	1,863	1,899
620-679 | Retail | Education
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	10	50
Year two	73	66
Year three	64	45
Year four	41	34
Year five	30	28
Prior to year five	121	99
Within the Revolving Period	0	0
Converted to Term	0	0
Total	339	322
620-679 | Retail | Other retail
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	37	149
Year two	129	119
Year three	99	43
Year four	36	19
Year five	16	7
Prior to year five	9	4
Within the Revolving Period	349	299
Converted to Term	5	5
Total	680	645
Less than 620 | Retail
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	38	225
Year two	383	258
Year three	283	364
Year four	359	310
Year five	289	252
Prior to year five	669	423
Within the Revolving Period	243	191
Converted to Term	190	182
Total	2,454	2,205
Less than 620 | Retail | Residential mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	0	24
Year two	45	66
Year three	82	164
Year four	171	162
Year five	158	157
Prior to year five	435	257
Within the Revolving Period	0	0
Converted to Term	0	0
Total	891	830
Less than 620 | Retail | Home equity
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	0	0
Year two	0	2
Year three	2	16
Year four	16	23
Year five	20	20
Prior to year five	103	87
Within the Revolving Period	114	91
Converted to Term	184	176
Total	439	415
Less than 620 | Retail | Automobile
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	32	164
Year two	291	142
Year three	149	155
Year four	145	103
Year five	91	62
Prior to year five	75	32
Within the Revolving Period	0	0
Converted to Term	0	0
Total	783	658
Less than 620 | Retail | Education
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	1	5
Year two	9	11
Year three	14	12
Year four	12	12
Year five	12	10
Prior to year five	52	45
Within the Revolving Period	0	0
Converted to Term	0	0
Total	100	95
Less than 620 | Retail | Other retail
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	5	32
Year two	38	37
Year three	36	17
Year four	15	10
Year five	8	3
Prior to year five	4	2
Within the Revolving Period	129	100
Converted to Term	6	6
Total	241	207
No FICO available | Retail
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	41	54
Year two	10	13
Year three	8	1
Year four	4	0
Year five	5	0
Prior to year five	68	62
Within the Revolving Period	381	330
Converted to Term	1	1
Total	518	461
No FICO available | Retail | Residential mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	0	3
Year two	5	8
Year three	3	1
Year four	4	0
Year five	5	0
Prior to year five	18	10
Within the Revolving Period	0	0
Converted to Term	0	0
Total	35	22
No FICO available | Retail | Automobile
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	2	3
Year two	1	0
Year three	0	0
Year four	0	0
Year five	0	0
Prior to year five	0	0
Within the Revolving Period	0	0
Converted to Term	0	0
Total	3	3
No FICO available | Retail | Education
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	2	4
Year two	1	0
Year three	0	0
Year four	0	0
Year five	0	0
Prior to year five	50	52
Within the Revolving Period	0	0
Converted to Term	0	0
Total	53	56
No FICO available | Retail | Other retail
Financing Receivable, Credit Quality Indicator [Line Items]
Year one	37	44
Year two	3	5
Year three	5	0
Year four	0	0
Year five	0	0
Prior to year five	0	0
Within the Revolving Period	381	330
Converted to Term	1	1
Total	$ 427	$ 380