LOANS - Credit Quality Indicators (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	$ 665,988	$ 675,883
Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	274,034	288,839
Consumer | Residential first mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	45,739	47,778
Consumer | Credit cards
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	120,518	129,813
Consumer | Home equity loans
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	6,638	7,128
Consumer | Personal, small business and other
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	4,600	4,509
Consumer | Less than or equal to 80%
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	47,729	49,851
Consumer | Less than or equal to 80% | Residential first mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year	1,958	11,447
One year prior to current fiscal year	11,401	7,029
Two years prior to current fiscal year	6,093	1,617
Three years prior to current fiscal year	1,474	2,711
Four years prior to current fiscal year	2,449	5,423
Prior	18,084	14,966
Loans, net of unearned income	41,459	43,193
Consumer | Less than or equal to 80% | Home equity loans (pre-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	2,684	2,876
Consumer | Less than or equal to 80% | Home equity loans (post-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	3,586	3,782
Consumer | Less than or equal to 80% | Home equity loans
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	6,270	6,658
Consumer | 80% but less than or equal to 100%
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	2,947	3,185
Consumer | 80% but less than or equal to 100% | Residential first mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year	443	1,543
One year prior to current fiscal year	1,184	376
Two years prior to current fiscal year	377	507
Three years prior to current fiscal year	392	269
Four years prior to current fiscal year	192	84
Prior	98	66
Loans, net of unearned income	2,686	2,845
Consumer | 80% but less than or equal to 100% | Home equity loans (pre-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	50	50
Consumer | 80% but less than or equal to 100% | Home equity loans (post-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	211	290
Consumer | 80% but less than or equal to 100% | Home equity loans
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	261	340
Consumer | Greater than 100%
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	91	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	8	4
Four years prior to current fiscal year	3	2
Prior	17	16
Loans, net of unearned income	31	35
Consumer | Greater than 100% | Home equity loans (pre-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	15	16
Consumer | Greater than 100% | Home equity loans (post-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	45	58
Consumer | Greater than 100% | Home equity loans
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	60	74
Consumer | LTV not available
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	1,610	1,761
Consumer | LTV not available | Residential first mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	1,563	1,705
Consumer | LTV not available | Home equity loans
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	47	56
Consumer | Total
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	52,377	54,906
Consumer | Less than 680
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	27,324	30,920
Consumer | Less than 680 | Residential first mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year	21	187
One year prior to current fiscal year	195	150
Two years prior to current fiscal year	131	246
Three years prior to current fiscal year	233	298
Four years prior to current fiscal year	286	323
Prior	1,905	1,708
Loans, net of unearned income	2,771	2,912
Consumer | Less than 680 | Credit cards
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	22,931	26,227
Consumer | Less than 680 | Credit cards | CANADA
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	530	572
Consumer | Less than 680 | Home equity loans (pre-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	272	292
Consumer | Less than 680 | Home equity loans (post-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	965	1,055
Consumer | Less than 680 | Home equity loans
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	1,237	1,347
Consumer | Less than 680 | Personal, small business and other
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year	1	23
One year prior to current fiscal year	26	79
Two years prior to current fiscal year	70	82
Three years prior to current fiscal year	67	26
Four years prior to current fiscal year	20	10
Prior	201	214
Loans, net of unearned income	385	434
Consumer | 680 to 760
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	64,083	68,634
Consumer | 680 to 760 | Residential first mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year	730	3,741
One year prior to current fiscal year	3,418	1,857
Two years prior to current fiscal year	1,639	655
Three years prior to current fiscal year	547	846
Four years prior to current fiscal year	740	1,368
Prior	4,880	4,133
Loans, net of unearned income	11,954	12,600
Consumer | 680 to 760 | Credit cards
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	49,139	52,778
Consumer | 680 to 760 | Home equity loans (pre-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	929	1,014
Consumer | 680 to 760 | Home equity loans (post-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	1,439	1,569
Consumer | 680 to 760 | Home equity loans
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	2,368	2,583
Consumer | 680 to 760 | Personal, small business and other
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year	6	58
One year prior to current fiscal year	65	106
Two years prior to current fiscal year	90	80
Three years prior to current fiscal year	66	27
Four years prior to current fiscal year	21	9
Prior	374	393
Loans, net of unearned income	622	673
Consumer | Greater than 760
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	79,318	84,224
Consumer | Greater than 760 | Residential first mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year	1,650	9,052
One year prior to current fiscal year	8,962	5,384
Two years prior to current fiscal year	4,700	1,227
Three years prior to current fiscal year	1,089	1,829
Four years prior to current fiscal year	1,612	3,799
Prior	11,365	9,105
Loans, net of unearned income	29,378	30,396
Consumer | Greater than 760 | Credit cards
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	46,084	49,767
Consumer | Greater than 760 | Home equity loans (pre-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	1,568	1,657
Consumer | Greater than 760 | Home equity loans (post-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	1,455	1,524
Consumer | Greater than 760 | Home equity loans
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	3,023	3,181
Consumer | Greater than 760 | Personal, small business and other
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year	13	95
One year prior to current fiscal year	112	134
Two years prior to current fiscal year	114	84
Three years prior to current fiscal year	70	30
Four years prior to current fiscal year	23	8
Prior	501	529
Loans, net of unearned income	833	880
Consumer | FICO not available
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	6,770	5,450
Consumer | FICO not available | Residential first mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	1,636	1,870
Consumer | FICO not available | Credit cards
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	2,364	1,041
Consumer | FICO not available | Home equity loans
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	10	17
Consumer | FICO not available | Personal, small business and other
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	2,760	2,522
Consumer | Total loans
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	$ 177,495	$ 189,228