ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Accruing and Nonaccruing Past Due Amounts (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Financing Receivables, 30 to 59 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	$ 392	$ 493
Financing Receivables 60 To 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	273	177
Financing Receivables 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Past Due	418	587
Financing Receivables, 30 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Past Due	1,083	1,257
Commercial | Financing Receivables, 30 to 59 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	24	100
Commercial | Financing Receivables, 30 to 59 Days Past Due | Commercial
Financing Receivable, Past Due [Line Items]
Past Due	17	85
Commercial | Financing Receivables, 30 to 59 Days Past Due | Commercial real estate
Financing Receivable, Past Due [Line Items]
Past Due	7	8
Commercial | Financing Receivables, 30 to 59 Days Past Due | Leases
Financing Receivable, Past Due [Line Items]
Past Due	0	7
Commercial | Financing Receivables 60 To 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	20	35
Commercial | Financing Receivables 60 To 89 Days Past Due | Commercial
Financing Receivable, Past Due [Line Items]
Past Due	4	3
Commercial | Financing Receivables 60 To 89 Days Past Due | Commercial real estate
Financing Receivable, Past Due [Line Items]
Past Due	16	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	78	83
Commercial | Financing Receivables 90 Days or More Past Due | Commercial
Financing Receivable, Past Due [Line Items]
Past Due	76	78
Commercial | Financing Receivables 90 Days or More Past Due | Commercial real estate
Financing Receivable, Past Due [Line Items]
Past Due	1	5
Commercial | Financing Receivables 90 Days or More Past Due | Leases
Financing Receivable, Past Due [Line Items]
Past Due	1	0
Commercial | Financing Receivables, 30 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Past Due	122	218
Commercial | Financing Receivables, 30 Days or More Past Due | Commercial
Financing Receivable, Past Due [Line Items]
Past Due	97	166
Commercial | Financing Receivables, 30 Days or More Past Due | Commercial real estate
Financing Receivable, Past Due [Line Items]
Past Due	24	45
Commercial | Financing Receivables, 30 Days or More Past Due | Leases
Financing Receivable, Past Due [Line Items]
Past Due	1	7
Retail | Financing Receivables, 30 to 59 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	368	393
Retail | Financing Receivables, 30 to 59 Days Past Due | Residential mortgages
Financing Receivable, Past Due [Line Items]
Past Due	28	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	52	65
Retail | Financing Receivables, 30 to 59 Days Past Due | Home equity loans serviced by others
Financing Receivable, Past Due [Line Items]
Past Due	5	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	194	207
Retail | Financing Receivables, 30 to 59 Days Past Due | Education
Financing Receivable, Past Due [Line Items]
Past Due	27	23
Retail | Financing Receivables, 30 to 59 Days Past Due | Credit cards
Financing Receivable, Past Due [Line Items]
Past Due	20	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	253	142
Retail | Financing Receivables 60 To 89 Days Past Due | Residential mortgages
Financing Receivable, Past Due [Line Items]
Past Due	78	13
Retail | Financing Receivables 60 To 89 Days Past Due | Home equity loans
Financing Receivable, Past Due [Line Items]
Past Due	25	3
Retail | Financing Receivables 60 To 89 Days Past Due | Home equity lines of credit
Financing Receivable, Past Due [Line Items]
Past Due	27	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	71	59
Retail | Financing Receivables 60 To 89 Days Past Due | Education
Financing Receivable, Past Due [Line Items]
Past Due	16	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	21	18
Retail | Financing Receivables 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Past Due	340	504
Retail | Financing Receivables 90 Days or More Past Due | Residential mortgages
Financing Receivable, Past Due [Line Items]
Past Due	89	133
Retail | Financing Receivables 90 Days or More Past Due | Home equity loans
Financing Receivable, Past Due [Line Items]
Past Due	7	38
Retail | Financing Receivables 90 Days or More Past Due | Home equity lines of credit
Financing Receivable, Past Due [Line Items]
Past Due	145	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	11	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	11	11
Retail | Financing Receivables 90 Days or More Past Due | Credit cards
Financing Receivable, Past Due [Line Items]
Past Due	21	20
Retail | Financing Receivables 90 Days or More Past Due | Other retail
Financing Receivable, Past Due [Line Items]
Past Due	20	15
Retail | Financing Receivables, 30 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Past Due	961	1,039
Retail | Financing Receivables, 30 Days or More Past Due | Residential mortgages
Financing Receivable, Past Due [Line Items]
Past Due	195	183
Retail | Financing Receivables, 30 Days or More Past Due | Home equity loans
Financing Receivable, Past Due [Line Items]
Past Due	42	53
Retail | Financing Receivables, 30 Days or More Past Due | Home equity lines of credit
Financing Receivable, Past Due [Line Items]
Past Due	224	282
Retail | Financing Receivables, 30 Days or More Past Due | Home equity loans serviced by others
Financing Receivable, Past Due [Line Items]
Past Due	20	23
Retail | Financing Receivables, 30 Days or More Past Due | Home equity lines of credit serviced by others
Financing Receivable, Past Due [Line Items]
Past Due	14	10
Retail | Financing Receivables, 30 Days or More Past Due | Automobile
Financing Receivable, Past Due [Line Items]
Past Due	289	338
Retail | Financing Receivables, 30 Days or More Past Due | Education
Financing Receivable, Past Due [Line Items]
Past Due	54	47
Retail | Financing Receivables, 30 Days or More Past Due | Credit cards
Financing Receivable, Past Due [Line Items]
Past Due	52	44
Retail | Financing Receivables, 30 Days or More Past Due | Other retail
Financing Receivable, Past Due [Line Items]
Past Due	$ 71	$ 59