LOANS - Credit Quality Indicators (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	$ 664,764	$ 675,883
Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	264,250	288,839
Consumer | Residential first mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	44,345	47,778
Consumer | Credit cards
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	125,001	129,813
Consumer | Home equity loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	5,485	7,128
Consumer | Personal, small business and other
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	3,179	4,509
Consumer | Less than or equal to 80%
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	47,168	49,851
Consumer | Less than or equal to 80% | Residential first mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year	7,875	11,447
One year prior to current fiscal year	11,049	7,029
Two years prior to current fiscal year	4,913	1,617
Three years prior to current fiscal year	1,378	2,711
Four years prior to current fiscal year	2,117	5,423
Prior	14,556	14,966
Total loans, net of unearned income	41,888	43,193
Consumer | Less than or equal to 80% | Home equity loans (pre-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	2,441	2,876
Consumer | Less than or equal to 80% | Home equity loans (post-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	2,839	3,782
Consumer | Less than or equal to 80% | Home equity loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	5,280	6,658
Consumer | Greater than 80% but less than or equal to 100%
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	908	3,185
Consumer | Greater than 80% but less than or equal to 100% | Residential first mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year	355	1,543
One year prior to current fiscal year	137	376
Two years prior to current fiscal year	96	507
Three years prior to current fiscal year	117	269
Four years prior to current fiscal year	56	84
Prior	44	66
Total loans, net of unearned income	805	2,845
Consumer | Greater than 80% but less than or equal to 100% | Home equity loans (pre-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	34	50
Consumer | Greater than 80% but less than or equal to 100% | Home equity loans (post-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	69	290
Consumer | Greater than 80% but less than or equal to 100% | Home equity loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	103	340
Consumer | Greater than 100%
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	57	109
Consumer | Greater than 100% | Residential first mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year	0	0
One year prior to current fiscal year	0	2
Two years prior to current fiscal year	1	11
Three years prior to current fiscal year	6	4
Four years prior to current fiscal year	2	2
Prior	10	16
Total loans, net of unearned income	19	35
Consumer | Greater than 100% | Home equity loans (pre-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	10	16
Consumer | Greater than 100% | Home equity loans (post-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	28	58
Consumer | Greater than 100% | Home equity loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	38	74
Consumer | LTV not available
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	1,697	1,761
Consumer | LTV not available | Residential first mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	1,633	1,705
Consumer | LTV not available | Home equity loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	64	56
Consumer | Total
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	49,830	54,906
Consumer | Less than 680
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	25,250	30,920
Consumer | Less than 680 | Residential first mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year	132	187
One year prior to current fiscal year	180	150
Two years prior to current fiscal year	133	246
Three years prior to current fiscal year	207	298
Four years prior to current fiscal year	249	323
Prior	1,602	1,708
Total loans, net of unearned income	2,503	2,912
Consumer | Less than 680 | Credit cards
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	21,579	26,227
Consumer | Less than 680 | Credit cards | CANADA
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	525	572
Consumer | Less than 680 | Home equity loans (pre-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	228	292
Consumer | Less than 680 | Home equity loans (post-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	678	1,055
Consumer | Less than 680 | Home equity loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	906	1,347
Consumer | Less than 680 | Personal, small business and other
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year	19	23
One year prior to current fiscal year	23	79
Two years prior to current fiscal year	48	82
Three years prior to current fiscal year	41	26
Four years prior to current fiscal year	11	10
Prior	120	214
Total loans, net of unearned income	262	434
Consumer | 680 to 760
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	65,270	68,634
Consumer | 680 to 760 | Residential first mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year	2,555	3,741
One year prior to current fiscal year	2,949	1,857
Two years prior to current fiscal year	1,369	655
Three years prior to current fiscal year	468	846
Four years prior to current fiscal year	646	1,368
Prior	4,164	4,133
Total loans, net of unearned income	12,151	12,600
Consumer | 680 to 760 | Credit cards
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	50,734	52,778
Consumer | 680 to 760 | Home equity loans (pre-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	877	1,014
Consumer | 680 to 760 | Home equity loans (post-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	1,097	1,569
Consumer | 680 to 760 | Home equity loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	1,974	2,583
Consumer | 680 to 760 | Personal, small business and other
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year	63	58
One year prior to current fiscal year	50	106
Two years prior to current fiscal year	65	80
Three years prior to current fiscal year	45	27
Four years prior to current fiscal year	12	9
Prior	176	393
Total loans, net of unearned income	411	673
Consumer | Greater than 760
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	81,427	84,224
Consumer | Greater than 760 | Residential first mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year	5,535	9,052
One year prior to current fiscal year	8,046	5,384
Two years prior to current fiscal year	3,499	1,227
Three years prior to current fiscal year	820	1,829
Four years prior to current fiscal year	1,272	3,799
Prior	8,755	9,105
Total loans, net of unearned income	27,927	30,396
Consumer | Greater than 760 | Credit cards
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	50,451	49,767
Consumer | Greater than 760 | Home equity loans (pre-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	1,399	1,657
Consumer | Greater than 760 | Home equity loans (post-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	1,176	1,524
Consumer | Greater than 760 | Home equity loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	2,575	3,181
Consumer | Greater than 760 | Personal, small business and other
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year	114	95
One year prior to current fiscal year	77	134
Two years prior to current fiscal year	82	84
Three years prior to current fiscal year	47	30
Four years prior to current fiscal year	13	8
Prior	141	529
Total loans, net of unearned income	474	880
Consumer | FICO not available
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	6,063	5,450
Consumer | FICO not available | Residential first mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	1,764	1,870
Consumer | FICO not available | Credit cards
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	2,237	1,041
Consumer | FICO not available | Home equity loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	30	17
Consumer | FICO not available | Personal, small business and other
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	2,032	2,522
Consumer | Total loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	$ 178,010	$ 189,228