ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Recorded Investment in Retail Loans by Delinquency Status (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable, Past Due [Line Items]
Total loans and leases	$ 119,088	$ 116,660
Financing Receivables, 30 to 59 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	511	493
Financing Receivables, 60 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	212	177
Financing Receivables, 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Past Due	419	587
Retail
Financing Receivable, Past Due [Line Items]
Current	58,651	56,883
Total loans and leases	61,550	59,877
Retail | Residential mortgages
Financing Receivable, Past Due [Line Items]
Current	18,818	18,664
Total loans and leases	19,083	18,978
Retail | Home equity loans
Financing Receivable, Past Due [Line Items]
Current	713	945
Total loans and leases	812	1,073
Retail | Home equity lines of credit
Financing Receivable, Past Due [Line Items]
Current	11,383	12,042
Total loans and leases	11,979	12,710
Retail | Home equity loans serviced by others
Financing Receivable, Past Due [Line Items]
Current	244	355
Total loans and leases	289	399
Retail | Home equity lines of credit serviced by others
Financing Receivable, Past Due [Line Items]
Current	50	79
Total loans and leases	74	104
Retail | Automobile
Financing Receivable, Past Due [Line Items]
Current	10,787	10,729
Total loans and leases	12,120	12,106
Retail | Education
Financing Receivable, Past Due [Line Items]
Current	10,088	8,694
Total loans and leases	10,347	8,900
Retail | Credit cards
Financing Receivable, Past Due [Line Items]
Current	2,076	1,894
Total loans and leases	2,198	1,991
Retail | Other retail
Financing Receivable, Past Due [Line Items]
Current	4,492	3,481
Total loans and leases	4,648	3,616
Retail | Financing Receivables, 1 to 29 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	1,937	1,955
Retail | Financing Receivables, 1 to 29 Days Past Due | Residential mortgages
Financing Receivable, Past Due [Line Items]
Past Due	129	131
Retail | Financing Receivables, 1 to 29 Days Past Due | Home equity loans
Financing Receivable, Past Due [Line Items]
Past Due	64	75
Retail | Financing Receivables, 1 to 29 Days Past Due | Home equity lines of credit
Financing Receivable, Past Due [Line Items]
Past Due	346	386
Retail | Financing Receivables, 1 to 29 Days Past Due | Home equity loans serviced by others
Financing Receivable, Past Due [Line Items]
Past Due	23	21
Retail | Financing Receivables, 1 to 29 Days Past Due | Home equity lines of credit serviced by others
Financing Receivable, Past Due [Line Items]
Past Due	11	15
Retail | Financing Receivables, 1 to 29 Days Past Due | Automobile
Financing Receivable, Past Due [Line Items]
Past Due	1,001	1,039
Retail | Financing Receivables, 1 to 29 Days Past Due | Education
Financing Receivable, Past Due [Line Items]
Past Due	202	159
Retail | Financing Receivables, 1 to 29 Days Past Due | Credit cards
Financing Receivable, Past Due [Line Items]
Past Due	74	53
Retail | Financing Receivables, 1 to 29 Days Past Due | Other retail
Financing Receivable, Past Due [Line Items]
Past Due	87	76
Retail | Financing Receivables, 30 to 59 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	428	393
Retail | Financing Receivables, 30 to 59 Days Past Due | Residential mortgages
Financing Receivable, Past Due [Line Items]
Past Due	35	37
Retail | Financing Receivables, 30 to 59 Days Past Due | Home equity loans
Financing Receivable, Past Due [Line Items]
Past Due	10	12
Retail | Financing Receivables, 30 to 59 Days Past Due | Home equity lines of credit
Financing Receivable, Past Due [Line Items]
Past Due	72	65
Retail | Financing Receivables, 30 to 59 Days Past Due | Home equity loans serviced by others
Financing Receivable, Past Due [Line Items]
Past Due	7	7
Retail | Financing Receivables, 30 to 59 Days Past Due | Home equity lines of credit serviced by others
Financing Receivable, Past Due [Line Items]
Past Due	2	2
Retail | Financing Receivables, 30 to 59 Days Past Due | Automobile
Financing Receivable, Past Due [Line Items]
Past Due	227	207
Retail | Financing Receivables, 30 to 59 Days Past Due | Education
Financing Receivable, Past Due [Line Items]
Past Due	30	23
Retail | Financing Receivables, 30 to 59 Days Past Due | Credit cards
Financing Receivable, Past Due [Line Items]
Past Due	15	14
Retail | Financing Receivables, 30 to 59 Days Past Due | Other retail
Financing Receivable, Past Due [Line Items]
Past Due	30	26
Retail | Financing Receivables, 60 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	184	142
Retail | Financing Receivables, 60 to 89 Days Past Due | Residential mortgages
Financing Receivable, Past Due [Line Items]
Past Due	17	13
Retail | Financing Receivables, 60 to 89 Days Past Due | Home equity loans
Financing Receivable, Past Due [Line Items]
Past Due	4	3
Retail | Financing Receivables, 60 to 89 Days Past Due | Home equity lines of credit
Financing Receivable, Past Due [Line Items]
Past Due	32	22
Retail | Financing Receivables, 60 to 89 Days Past Due | Home equity loans serviced by others
Financing Receivable, Past Due [Line Items]
Past Due	3	3
Retail | Financing Receivables, 60 to 89 Days Past Due | Home equity lines of credit serviced by others
Financing Receivable, Past Due [Line Items]
Past Due	1	1
Retail | Financing Receivables, 60 to 89 Days Past Due | Automobile
Financing Receivable, Past Due [Line Items]
Past Due	81	59
Retail | Financing Receivables, 60 to 89 Days Past Due | Education
Financing Receivable, Past Due [Line Items]
Past Due	15	13
Retail | Financing Receivables, 60 to 89 Days Past Due | Credit cards
Financing Receivable, Past Due [Line Items]
Past Due	11	10
Retail | Financing Receivables, 60 to 89 Days Past Due | Other retail
Financing Receivable, Past Due [Line Items]
Past Due	20	18
Retail | Financing Receivables, 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Past Due	350	504
Retail | Financing Receivables, 90 Days or More Past Due | Residential mortgages
Financing Receivable, Past Due [Line Items]
Past Due	84	133
Retail | Financing Receivables, 90 Days or More Past Due | Home equity loans
Financing Receivable, Past Due [Line Items]
Past Due	21	38
Retail | Financing Receivables, 90 Days or More Past Due | Home equity lines of credit
Financing Receivable, Past Due [Line Items]
Past Due	146	195
Retail | Financing Receivables, 90 Days or More Past Due | Home equity loans serviced by others
Financing Receivable, Past Due [Line Items]
Past Due	12	13
Retail | Financing Receivables, 90 Days or More Past Due | Home equity lines of credit serviced by others
Financing Receivable, Past Due [Line Items]
Past Due	10	7
Retail | Financing Receivables, 90 Days or More Past Due | Automobile
Financing Receivable, Past Due [Line Items]
Past Due	24	72
Retail | Financing Receivables, 90 Days or More Past Due | Education
Financing Receivable, Past Due [Line Items]
Past Due	12	11
Retail | Financing Receivables, 90 Days or More Past Due | Credit cards
Financing Receivable, Past Due [Line Items]
Past Due	22	20
Retail | Financing Receivables, 90 Days or More Past Due | Other retail
Financing Receivable, Past Due [Line Items]
Past Due	$ 19	$ 15