LOANS - Credit Quality Indicators (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	$ 676,834	$ 675,883
Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	275,895	288,839
Consumer | Residential first mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	44,835	47,778
Consumer | Credit cards
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	124,295	129,813
Consumer | Home equity loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	6,168	7,128
Consumer | Personal, small business and other
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	3,676	4,509
Consumer | Less than or equal to 80%
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	47,429	49,851
Consumer | Less than or equal to 80% | Residential first mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year	4,698	11,447
One year prior to current fiscal year	11,573	7,029
Two years prior to current fiscal year	5,513	1,617
Three years prior to current fiscal year	1,398	2,711
Four years prior to current fiscal year	2,232	5,423
Prior	16,129	14,966
Total loans, net of unearned income	41,543	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	2,562	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	3,324	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	5,886	6,658
Consumer | 80% but less than or equal to 100%
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	1,747	3,185
Consumer | 80% but less than or equal to 100% | Residential first mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year	454	1,543
One year prior to current fiscal year	430	376
Two years prior to current fiscal year	211	507
Three years prior to current fiscal year	262	269
Four years prior to current fiscal year	130	84
Prior	73	66
Total loans, net of unearned income	1,560	2,845
Consumer | 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	42	50
Consumer | 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	145	290
Consumer | 80% but less than or equal to 100% | Home equity loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	187	340
Consumer | Greater than 100%
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	75	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	3	11
Three years prior to current fiscal year	7	4
Four years prior to current fiscal year	3	2
Prior	13	16
Total loans, net of unearned income	26	35
Consumer | Greater than 100% | Home equity loans (pre-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	14	16
Consumer | Greater than 100% | Home equity loans (post-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	35	58
Consumer | Greater than 100% | Home equity loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	49	74
Consumer | LTV not available
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	1,752	1,761
Consumer | LTV not available | Residential first mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	1,706	1,705
Consumer | LTV not available | Home equity loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	46	56
Consumer | Total
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	51,003	54,906
Consumer | Less than 680
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	24,923	30,920
Consumer | Less than 680 | Residential first mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year	40	187
One year prior to current fiscal year	213	150
Two years prior to current fiscal year	127	246
Three years prior to current fiscal year	208	298
Four years prior to current fiscal year	257	323
Prior	1,707	1,708
Total loans, net of unearned income	2,552	2,912
Consumer | Less than 680 | Credit cards
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	21,014	26,227
Consumer | Less than 680 | Credit cards | CANADA
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	528	572
Consumer | Less than 680 | Home equity loans (pre-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	237	292
Consumer | Less than 680 | Home equity loans (post-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	846	1,055
Consumer | Less than 680 | Home equity loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	1,083	1,347
Consumer | Less than 680 | Personal, small business and other
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year	8	23
One year prior to current fiscal year	24	79
Two years prior to current fiscal year	55	82
Three years prior to current fiscal year	51	26
Four years prior to current fiscal year	14	10
Prior	122	214
Total loans, net of unearned income	274	434
Consumer | 680 to 760
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	64,938	68,634
Consumer | 680 to 760 | Residential first mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year	1,465	3,741
One year prior to current fiscal year	3,175	1,857
Two years prior to current fiscal year	1,527	655
Three years prior to current fiscal year	533	846
Four years prior to current fiscal year	697	1,368
Prior	4,593	4,133
Total loans, net of unearned income	11,990	12,600
Consumer | 680 to 760 | Credit cards
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	50,253	52,778
Consumer | 680 to 760 | Home equity loans (pre-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	932	1,014
Consumer | 680 to 760 | Home equity loans (post-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	1,347	1,569
Consumer | 680 to 760 | Home equity loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	2,279	2,583
Consumer | 680 to 760 | Personal, small business and other
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year	35	58
One year prior to current fiscal year	56	106
Two years prior to current fiscal year	77	80
Three years prior to current fiscal year	55	27
Four years prior to current fiscal year	16	9
Prior	177	393
Total loans, net of unearned income	416	673
Consumer | Greater than 760
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	82,547	84,224
Consumer | Greater than 760 | Residential first mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year	3,637	9,052
One year prior to current fiscal year	8,605	5,384
Two years prior to current fiscal year	4,063	1,227
Three years prior to current fiscal year	919	1,829
Four years prior to current fiscal year	1,403	3,799
Prior	9,816	9,105
Total loans, net of unearned income	28,443	30,396
Consumer | Greater than 760 | Credit cards
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	50,827	49,767
Consumer | Greater than 760 | Home equity loans (pre-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	1,470	1,657
Consumer | Greater than 760 | Home equity loans (post-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	1,327	1,524
Consumer | Greater than 760 | Home equity loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	2,797	3,181
Consumer | Greater than 760 | Personal, small business and other
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year	68	95
One year prior to current fiscal year	95	134
Two years prior to current fiscal year	99	84
Three years prior to current fiscal year	58	30
Four years prior to current fiscal year	18	8
Prior	142	529
Total loans, net of unearned income	480	880
Consumer | FICO not available
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	6,566	5,450
Consumer | FICO not available | Residential first mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	1,850	1,870
Consumer | FICO not available | Credit cards
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	2,201	1,041
Consumer | FICO not available | Home equity loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	9	17
Consumer | FICO not available | Personal, small business and other
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	2,506	2,522
Consumer | Total loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans, net of unearned income	$ 178,974	$ 189,228