ALLOWANCE FOR CREDIT LOSSES, NONACCRUING LOANS AND LEASES, AND CONCENTRATIONS OF CREDIT RISK - Amortized Cost Basis by Credit Quality Indicator (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Financing Receivable, Credit Quality Indicator [Line Items]
Total	$ 122,581	$ 123,090
SBA Guaranteed PPP Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total		4,200
SBA Guaranteed PPP Loans \| Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total	3,500
Commercial
Financing Receivable, Credit Quality Indicator [Line Items]
2021	6,014	10,683
2020	8,335	9,445
2019	10,102	7,862
2018	7,564	3,746
2017	3,605	2,200
Prior to 2017	5,568	3,998
Within the Revolving Period	17,720	22,459
Converted to Term	175	400
Total	59,083	60,793
Commercial \| Pass
Financing Receivable, Credit Quality Indicator [Line Items]
2021	5,877	10,339
2020	8,080	8,812
2019	9,202	7,219
2018	6,839	3,419
2017	3,099	1,903
Prior to 2017	4,926	3,572
Within the Revolving Period	16,662	20,552
Converted to Term	151	340
Total	54,836	56,156
Commercial \| Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
2021	77	56
2020	50	398
2019	390	286
2018	300	180
2017	234	156
Prior to 2017	321	239
Within the Revolving Period	468	1,071
Converted to Term	0	34
Total	1,840	2,420
Commercial \| Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
2021	33	207
2020	180	199
2019	478	315
2018	403	109
2017	260	138
Prior to 2017	300	153
Within the Revolving Period	583	749
Converted to Term	21	22
Total	2,258	1,892
Commercial \| Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
2021	27	81
2020	25	36
2019	32	42
2018	22	38
2017	12	3
Prior to 2017	21	34
Within the Revolving Period	7	87
Converted to Term	3	4
Total	149	325
Commercial \| Commercial and industrial
Financing Receivable, Credit Quality Indicator [Line Items]
2021	5,208	8,226
2020	5,322	6,199
2019	5,735	4,625
2018	4,108	2,396
2017	2,175	1,301
Prior to 2017	3,222	2,311
Within the Revolving Period	16,897	18,715
Converted to Term	175	400
Total	42,842	44,173
Commercial \| Commercial and industrial \| Pass
Financing Receivable, Credit Quality Indicator [Line Items]
2021	5,146	8,036
2020	5,140	5,730
2019	5,260	4,180
2018	3,623	2,174
2017	1,975	1,157
Prior to 2017	2,797	1,980
Within the Revolving Period	15,853	17,281
Converted to Term	151	340
Total	39,945	40,878
Commercial \| Commercial and industrial \| Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
2021	3	34
2020	41	264
2019	196	163
2018	196	84
2017	74	60
Prior to 2017	181	173
Within the Revolving Period	454	771
Converted to Term	0	34
Total	1,145	1,583
Commercial \| Commercial and industrial \| Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
2021	32	91
2020	125	195
2019	263	248
2018	267	100
2017	114	81
Prior to 2017	226	127
Within the Revolving Period	583	600
Converted to Term	21	22
Total	1,631	1,464
Commercial \| Commercial and industrial \| Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
2021	27	65
2020	16	10
2019	16	34
2018	22	38
2017	12	3
Prior to 2017	18	31
Within the Revolving Period	7	63
Converted to Term	3	4
Total	121	248
Commercial \| Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
2021	536	1,999
2020	2,627	2,994
2019	4,145	3,004
2018	3,250	1,203
2017	1,327	713
Prior to 2017	1,704	995
Within the Revolving Period	823	3,744
Converted to Term	0	0
Total	14,412	14,652
Commercial \| Commercial real estate \| Pass
Financing Receivable, Credit Quality Indicator [Line Items]
2021	462	1,848
2020	2,572	2,836
2019	3,726	2,810
2018	3,013	1,106
2017	1,026	566
Prior to 2017	1,507	919
Within the Revolving Period	809	3,271
Converted to Term	0	0
Total	13,115	13,356
Commercial \| Commercial real estate \| Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
2021	73	19
2020	7	130
2019	193	121
2018	102	92
2017	155	94
Prior to 2017	122	48
Within the Revolving Period	14	300
Converted to Term	0	0
Total	666	804
Commercial \| Commercial real estate \| Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
2021	1	116
2020	39	2
2019	210	65
2018	135	5
2017	146	53
Prior to 2017	73	26
Within the Revolving Period	0	149
Converted to Term	0	0
Total	604	416
Commercial \| Commercial real estate \| Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
2021	0	16
2020	9	26
2019	16	8
2018	0	0
2017	0	0
Prior to 2017	2	2
Within the Revolving Period	0	24
Converted to Term	0	0
Total	27	76
Commercial \| Leases
Financing Receivable, Credit Quality Indicator [Line Items]
2021	270	458
2020	386	252
2019	222	233
2018	206	147
2017	103	186
Prior to 2017	642	692
Within the Revolving Period	0	0
Converted to Term	0	0
Total	1,829	1,968
Commercial \| Leases \| Pass
Financing Receivable, Credit Quality Indicator [Line Items]
2021	269	455
2020	368	246
2019	216	229
2018	203	139
2017	98	180
Prior to 2017	622	673
Within the Revolving Period	0	0
Converted to Term	0	0
Total	1,776	1,922
Commercial \| Leases \| Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
2021	1	3
2020	2	4
2019	1	2
2018	2	4
2017	5	2
Prior to 2017	18	18
Within the Revolving Period	0	0
Converted to Term	0	0
Total	29	33
Commercial \| Leases \| Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
2021	0	0
2020	16	2
2019	5	2
2018	1	4
2017	0	4
Prior to 2017	1	0
Within the Revolving Period	0	0
Converted to Term	0	0
Total	23	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
Commercial \| SBA Guaranteed PPP Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total	3,500
Retail
Financing Receivable, Credit Quality Indicator [Line Items]
2021	8,316	17,242
2020	15,883	10,972
2019	9,030	5,335
2018	4,385	4,816
2017	3,970	4,462
Prior to 2017	8,806	6,323
Within the Revolving Period	11,819	11,755
Converted to Term	1,289	1,392
Total	63,498	62,297
Retail \| Residential mortgages (2)
Financing Receivable, Credit Quality Indicator [Line Items]
2021	2,893	6,512
2020	6,157	3,493
2019	3,158	1,278
2018	1,208	1,947
2017	1,798	2,770
Prior to 2017	5,324	3,539
Within the Revolving Period	0	0
Converted to Term	0	0
Total	20,538	19,539
Retail \| Home equity
Financing Receivable, Credit Quality Indicator [Line Items]
2021	1	6
2020	10	47
2019	52	75
2018	74	78
2017	72	50
Prior to 2017	717	833
Within the Revolving Period	9,648	9,692
Converted to Term	1,267	1,368
Total	11,841	12,149
Retail \| Automobile
Financing Receivable, Credit Quality Indicator [Line Items]
2021	3,341	4,728
2020	4,024	3,449
2019	2,709	1,864
2018	1,386	1,232
2017	853	642
Prior to 2017	467	238
Within the Revolving Period	0	0
Converted to Term	0	0
Total	12,780	12,153
Retail \| Education
Financing Receivable, Credit Quality Indicator [Line Items]
2021	1,544	4,094
2020	4,270	2,706
2019	2,339	1,606
2018	1,363	1,333
2017	1,090	952
Prior to 2017	2,194	1,617
Within the Revolving Period	0	0
Converted to Term	0	0
Total	12,800	12,308
Retail \| Other retail
Financing Receivable, Credit Quality Indicator [Line Items]
2021	537	1,902
2020	1,422	1,277
2019	772	512
2018	354	226
2017	157	48
Prior to 2017	104	96
Within the Revolving Period	2,171	2,063
Converted to Term	22	24
Total	5,539	6,148
Greater than 800 \| Retail
Financing Receivable, Credit Quality Indicator [Line Items]
2021	2,279	6,023
2020	6,218	4,448
2019	3,573	2,084
2018	1,555	2,306
2017	1,763	2,382
Prior to 2017	4,097	2,854
Within the Revolving Period	4,649	4,660
Converted to Term	318	344
Total	24,452	25,101
Greater than 800 \| Retail \| Residential mortgages (2)
Financing Receivable, Credit Quality Indicator [Line Items]
2021	851	2,687
2020	3,079	1,885
2019	1,574	638
2018	454	1,129
2017	897	1,615
Prior to 2017	2,697	1,755
Within the Revolving Period	0	0
Converted to Term	0	0
Total	9,552	9,709
Greater than 800 \| Retail \| Home equity
Financing Receivable, Credit Quality Indicator [Line Items]
2021	1	2
2020	2	8
2019	6	10
2018	6	7
2017	5	5
Prior to 2017	170	216
Within the Revolving Period	4,292	4,319
Converted to Term	318	344
Total	4,800	4,911
Greater than 800 \| Retail \| Automobile
Financing Receivable, Credit Quality Indicator [Line Items]
2021	756	1,056
2020	951	812
2019	681	424
2018	333	312
2017	223	169
Prior to 2017	122	62
Within the Revolving Period	0	0
Converted to Term	0	0
Total	3,066	2,835
Greater than 800 \| Retail \| Education
Financing Receivable, Credit Quality Indicator [Line Items]
2021	564	1,817
2020	1,843	1,363
2019	1,103	849
2018	662	781
2017	590	578
Prior to 2017	1,066	777
Within the Revolving Period	0	0
Converted to Term	0	0
Total	5,828	6,165
Greater than 800 \| Retail \| Other retail
Financing Receivable, Credit Quality Indicator [Line Items]
2021	107	461
2020	343	380
2019	209	163
2018	100	77
2017	48	15
Prior to 2017	42	44
Within the Revolving Period	357	341
Converted to Term	0	0
Total	1,206	1,481
740-799 \| Retail
Financing Receivable, Credit Quality Indicator [Line Items]
2021	3,643	6,864
2020	5,892	3,630
2019	2,885	1,661
2018	1,326	1,345
2017	1,092	1,190
Prior to 2017	2,253	1,597
Within the Revolving Period	3,995	3,872
Converted to Term	308	333
Total	21,394	20,492
740-799 \| Retail \| Residential mortgages (2)
Financing Receivable, Credit Quality Indicator [Line Items]
2021	1,619	2,931
2020	2,261	1,133
2019	890	398
2018	311	527
2017	448	743
Prior to 2017	1,335	904
Within the Revolving Period	0	0
Converted to Term	0	0
Total	6,864	6,636
740-799 \| Retail \| Home equity
Financing Receivable, Credit Quality Indicator [Line Items]
2021	0	2
2020	1	6
2019	5	7
2018	6	6
2017	6	5
Prior to 2017	146	180
Within the Revolving Period	3,333	3,234
Converted to Term	306	331
Total	3,803	3,771
740-799 \| Retail \| Automobile
Financing Receivable, Credit Quality Indicator [Line Items]
2021	1,096	1,514
2020	1,320	1,022
2019	813	531
2018	401	344
2017	242	172
Prior to 2017	122	59
Within the Revolving Period	0	0
Converted to Term	0	0
Total	3,994	3,642
740-799 \| Retail \| Education
Financing Receivable, Credit Quality Indicator [Line Items]
2021	759	1,797
2020	1,831	1,009
2019	892	541
2018	480	387
2017	338	251
Prior to 2017	614	423
Within the Revolving Period	0	0
Converted to Term	0	0
Total	4,914	4,408
740-799 \| Retail \| Other retail
Financing Receivable, Credit Quality Indicator [Line Items]
2021	169	620
2020	479	460
2019	285	184
2018	128	81
2017	58	19
Prior to 2017	36	31
Within the Revolving Period	662	638
Converted to Term	2	2
Total	1,819	2,035
680-739 \| Retail
Financing Receivable, Credit Quality Indicator [Line Items]
2021	1,700	3,077
2020	2,671	1,842
2019	1,537	917
2018	783	644
2017	537	541
Prior to 2017	1,238	918
Within the Revolving Period	2,201	2,193
Converted to Term	279	289
Total	10,946	10,421
680-739 \| Retail \| Residential mortgages (2)
Financing Receivable, Credit Quality Indicator [Line Items]
2021	377	784
2020	653	351
2019	360	162
2018	178	172
2017	169	295
Prior to 2017	660	458
Within the Revolving Period	0	0
Converted to Term	0	0
Total	2,397	2,222
680-739 \| Retail \| Home equity
Financing Receivable, Credit Quality Indicator [Line Items]
2021	0	1
2020	1	6
2019	8	10
2018	13	15
2017	18	8
Prior to 2017	162	179
Within the Revolving Period	1,608	1,632
Converted to Term	274	284
Total	2,084	2,135
680-739 \| Retail \| Automobile
Financing Receivable, Credit Quality Indicator [Line Items]
2021	969	1,347
2020	1,109	889
2019	690	461
2018	335	282
2017	190	138
Prior to 2017	98	47
Within the Revolving Period	0	0
Converted to Term	0	0
Total	3,391	3,164
680-739 \| Retail \| Education
Financing Receivable, Credit Quality Indicator [Line Items]
2021	204	450
2020	536	294
2019	289	173
2018	172	127
2017	123	90
Prior to 2017	300	221
Within the Revolving Period	0	0
Converted to Term	0	0
Total	1,624	1,355
680-739 \| Retail \| Other retail
Financing Receivable, Credit Quality Indicator [Line Items]
2021	150	495
2020	372	302
2019	190	111
2018	85	48
2017	37	10
Prior to 2017	18	13
Within the Revolving Period	593	561
Converted to Term	5	5
Total	1,450	1,545
620-679 \| Retail
Financing Receivable, Credit Quality Indicator [Line Items]
2021	608	1,040
2020	856	727
2019	648	391

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