ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Accruing and Nonaccruing Past Due Amounts (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
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
Financing Receivables, Equal To Or Greater Than 30 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	1,142	1,257
Commercial | Financing Receivables, 30 to 59 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	83	100
Commercial | Financing Receivables, 30 to 59 Days Past Due | Commercial
Financing Receivable, Past Due [Line Items]
Past Due	45	85
Commercial | Financing Receivables, 30 to 59 Days Past Due | Commercial real estate
Financing Receivable, Past Due [Line Items]
Past Due	1	8
Commercial | Financing Receivables, 30 to 59 Days Past Due | Leases
Financing Receivable, Past Due [Line Items]
Past Due	37	7
Commercial | Financing Receivables, 60 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	28	35
Commercial | Financing Receivables, 60 to 89 Days Past Due | Commercial
Financing Receivable, Past Due [Line Items]
Past Due	27	3
Commercial | Financing Receivables, 60 to 89 Days Past Due | Commercial real estate
Financing Receivable, Past Due [Line Items]
Past Due	1	32
Commercial | Financing Receivables, 60 to 89 Days Past Due | Leases
Financing Receivable, Past Due [Line Items]
Past Due	0	0
Commercial | Financing Receivables, 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Past Due	69	83
Commercial | Financing Receivables, 90 Days or More Past Due | Commercial
Financing Receivable, Past Due [Line Items]
Past Due	67	78
Commercial | Financing Receivables, 90 Days or More Past Due | Commercial real estate
Financing Receivable, Past Due [Line Items]
Past Due	0	5
Commercial | Financing Receivables, 90 Days or More Past Due | Leases
Financing Receivable, Past Due [Line Items]
Past Due	2	0
Commercial | Financing Receivables, Equal To Or Greater Than 30 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	180	218
Commercial | Financing Receivables, Equal To Or Greater Than 30 Days Past Due | Commercial
Financing Receivable, Past Due [Line Items]
Past Due	139	166
Commercial | Financing Receivables, Equal To Or Greater Than 30 Days Past Due | Commercial real estate
Financing Receivable, Past Due [Line Items]
Past Due	2	45
Commercial | Financing Receivables, Equal To Or Greater Than 30 Days Past Due | Leases
Financing Receivable, Past Due [Line Items]
Past Due	39	7
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
Retail | Financing Receivables, Equal To Or Greater Than 30 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	962	1,039
Retail | Financing Receivables, Equal To Or Greater Than 30 Days Past Due | Residential mortgages
Financing Receivable, Past Due [Line Items]
Past Due	136	183
Retail | Financing Receivables, Equal To Or Greater Than 30 Days Past Due | Home equity loans
Financing Receivable, Past Due [Line Items]
Past Due	35	53
Retail | Financing Receivables, Equal To Or Greater Than 30 Days Past Due | Home equity lines of credit
Financing Receivable, Past Due [Line Items]
Past Due	250	282
Retail | Financing Receivables, Equal To Or Greater Than 30 Days Past Due | Home equity loans serviced by others
Financing Receivable, Past Due [Line Items]
Past Due	22	23
Retail | Financing Receivables, Equal To Or Greater Than 30 Days Past Due | Home equity lines of credit serviced by others
Financing Receivable, Past Due [Line Items]
Past Due	13	10
Retail | Financing Receivables, Equal To Or Greater Than 30 Days Past Due | Automobile
Financing Receivable, Past Due [Line Items]
Past Due	332	338
Retail | Financing Receivables, Equal To Or Greater Than 30 Days Past Due | Education
Financing Receivable, Past Due [Line Items]
Past Due	57	47
Retail | Financing Receivables, Equal To Or Greater Than 30 Days Past Due | Credit cards
Financing Receivable, Past Due [Line Items]
Past Due	48	44
Retail | Financing Receivables, Equal To Or Greater Than 30 Days Past Due | Other retail
Financing Receivable, Past Due [Line Items]
Past Due	$ 69	$ 59