LOANS - Credit Quality Indicators (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	$ 721,020	$ 699,483
Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	288,430	309,548
Consumer | Less than or equal to 80%
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	49,240	49,639
Consumer | Less than or equal to 80% | Residential first mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1,706
2019	7,825
2018	2,310
2017	3,307
2016	6,851
Prior	19,646
Loans, net of unearned income	41,645	41,705
Consumer | Less than or equal to 80% | Home equity loans (pre-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	3,034
Consumer | Less than or equal to 80% | Home equity loans (post-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	4,561
Consumer | Less than or equal to 80% | Home equity loans
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	7,595	7,934
Consumer | 80% but less than or equal to 100%
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	4,441	4,121
Consumer | 80% but less than or equal to 100% | Residential first mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
2020	363
2019	1,624
2018	736
2017	547
2016	208
Prior	179
Loans, net of unearned income	3,657	3,302
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	86
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	698
Consumer | 80% but less than or equal to 100% | Home equity loans
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	784	819
Consumer | Greater than 100%
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	308	333
Consumer | Greater than 100% | Residential first mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	5
2018	39
2017	10
2016	5
Prior	26
Loans, net of unearned income	85	98
Consumer | Greater than 100% | Home equity loans (pre-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	13
Consumer | Greater than 100% | Home equity loans (post-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	210
Consumer | Greater than 100% | Home equity loans
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	223	235
Consumer | Less than 680
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	38,889	39,337
Consumer | Less than 680 | Residential first mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
2020	33
2019	229
2018	314
2017	366
2016	417
Prior	2,247
Loans, net of unearned income	3,606	3,602
Consumer | Less than 680 | Credit cards
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	32,912	33,290
Consumer | Less than 680 | Credit cards and line-of-credit arrangements converted to term loans
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	0
Consumer | Less than 680 | Home equity loans (pre-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	342
Consumer | Less than 680 | Home equity loans (post-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	1,491
Consumer | Less than 680 | Home equity loans
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	1,833	1,881
Consumer | Less than 680 | Personal, small business and other
Financing Receivable, Credit Quality Indicator [Line Items]
2020	28
2019	133
2018	122
2017	38
2016	20
Prior	197
Loans, net of unearned income	538	564
Consumer | 680 to 760
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	74,127	77,096
Consumer | 680 to 760 | Residential first mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
2020	699
2019	3,054
2018	1,010
2017	1,165
2016	1,823
Prior	5,425
Loans, net of unearned income	13,176	13,178
Consumer | 680 to 760 | Credit cards
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	56,763	59,536
Consumer | 680 to 760 | Credit cards and line-of-credit arrangements converted to term loans
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	0
Consumer | 680 to 760 | Home equity loans (pre-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	1,189
Consumer | 680 to 760 | Home equity loans (post-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	2,130
Consumer | 680 to 760 | Home equity loans
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	3,319	3,475
Consumer | 680 to 760 | Personal, small business and other
Financing Receivable, Credit Quality Indicator [Line Items]
2020	53
2019	189
2018	173
2017	64
2016	30
Prior	360
Loans, net of unearned income	869	907
Consumer | Greater than 760
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	77,392	86,273
Consumer | Greater than 760 | Residential first mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1,337
2019	6,166
2018	1,754
2017	2,326
2016	4,814
Prior	12,131
Loans, net of unearned income	28,528	28,235
Consumer | Greater than 760 | Credit cards
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	44,486	52,935
Consumer | Greater than 760 | Credit cards and line-of-credit arrangements converted to term loans
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	0
Consumer | Greater than 760 | Home equity loans (pre-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	1,622
Consumer | Greater than 760 | Home equity loans (post-reset)
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	1,869
Consumer | Greater than 760 | Home equity loans
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, net of unearned income	3,491	3,630
Consumer | Greater than 760 | Personal, small business and other
Financing Receivable, Credit Quality Indicator [Line Items]
2020	48
2019	150
2018	105
2017	51
2016	20
Prior	513
Loans, net of unearned income	$ 887	$ 1,473