LOANS - Credit Quality Indicators (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	$ 666,911	$ 699,483
Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	280,025	309,548
Consumer | Residential first mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	48,370	47,008
Consumer | Credit cards
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	124,917	148,534
Consumer | Home equity loans
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	7,625	9,223
Consumer | Personal, small business and other
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	4,689	3,699
Consumer | Less than or equal to 80%
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	50,554	50,094
Consumer | Less than or equal to 80% | Residential first mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
2020	8,643
2019	7,437
2018	1,908
2017	2,988
2016	6,003
Prior	16,562
Loans, net of unearned income	43,541	41,993
Consumer | Less than or equal to 80% | Home equity loans (pre-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	2,991
Consumer | Less than or equal to 80% | Home equity loans (post-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	4,022
Consumer | Less than or equal to 80% | Home equity loans
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	7,013	8,101
Consumer | 80% but less than or equal to 100%
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	3,715	4,142
Consumer | 80% but less than or equal to 100% | Residential first mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1,515
2019	610
2018	584
2017	351
2016	108
Prior	97
Loans, net of unearned income	3,265	3,313
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	44
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	406
Consumer | 80% but less than or equal to 100% | Home equity loans
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	450	829
Consumer | Greater than 100%
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	159	335
Consumer | Greater than 100% | Residential first mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1
2019	3
2018	18
2017	5
2016	3
Prior	19
Loans, net of unearned income	49	98
Consumer | Greater than 100% | Home equity loans (pre-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	15
Consumer | Greater than 100% | Home equity loans (post-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	95
Consumer | Greater than 100% | Home equity loans
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	110	237
Consumer | LTV not available
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	1,567	1,660
Consumer | LTV not available | Residential first mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	1,515	1,604
Consumer | LTV not available | Home equity loans
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	52	56
Consumer | Total
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	55,995	56,231
Consumer | Less than 680
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	31,668	39,363
Consumer | Less than 680 | Residential first mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
2020	114
2019	177
2018	272
2017	331
2016	351
Prior	1,829
Loans, net of unearned income	3,074	3,608
Consumer | Less than 680 | Credit cards
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	26,613	33,290
Consumer | Less than 680 | Credit cards | CANADA
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	568	629
Consumer | Less than 680 | Home equity loans (pre-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	309
Consumer | Less than 680 | Home equity loans (post-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	1,179
Consumer | Less than 680 | Home equity loans
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	1,488	1,901
Consumer | Less than 680 | Personal, small business and other
Financing Receivable, Credit Quality Indicator [Line Items]
2020	21
2019	93
2018	100
2017	33
2016	14
Prior	232
Loans, net of unearned income	493	564
Consumer | 680 to 760
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	67,623	77,237
Consumer | 680 to 760 | Residential first mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
2020	2,945
2019	2,040
2018	789
2017	923
2016	1,482
Prior	4,361
Loans, net of unearned income	12,540	13,264
Consumer | 680 to 760 | Credit cards
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	51,604	59,536
Consumer | 680 to 760 | Home equity loans (pre-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	1,061
Consumer | 680 to 760 | Home equity loans (post-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	1,704
Consumer | 680 to 760 | Home equity loans
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	2,765	3,530
Consumer | 680 to 760 | Personal, small business and other
Financing Receivable, Credit Quality Indicator [Line Items]
2020	49
2019	121
2018	95
2017	33
2016	13
Prior	403
Loans, net of unearned income	714	907
Consumer | Greater than 760
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	80,634	86,582
Consumer | Greater than 760 | Residential first mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
2020	7,092
2019	5,827
2018	1,443
2017	2,084
2016	4,272
Prior	10,446
Loans, net of unearned income	31,164	28,442
Consumer | Greater than 760 | Credit cards
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	45,187	52,935
Consumer | Greater than 760 | Home equity loans (pre-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	1,702
Consumer | Greater than 760 | Home equity loans (post-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	1,656
Consumer | Greater than 760 | Home equity loans
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	3,358	3,732
Consumer | Greater than 760 | Personal, small business and other
Financing Receivable, Credit Quality Indicator [Line Items]
2020	76
2019	153
2018	97
2017	38
2016	12
Prior	549
Loans, net of unearned income	925	1,473
Consumer | FICO not available
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	5,676	5,282
Consumer | FICO not available | Residential first mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	1,592	1,694
Consumer | FICO not available | Credit cards
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	1,513	2,773
Consumer | FICO not available | Home equity loans
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	14	60
Consumer | FICO not available | Personal, small business and other
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	2,557	755
Consumer | Total loans
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	$ 185,601	$ 208,464