ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Accruing and Nonaccruing Past Due Amounts (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 1,036	$ 1,257
Financing Receivables, 30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	392	493
Financing Receivables 60 To 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	188	177
Financing Receivables 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	456	587
Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	90	218
Commercial | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	80	166
Commercial | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	8	45
Commercial | Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	2	7
Commercial | Financing Receivables, 30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	13	100
Commercial | Financing Receivables, 30 to 59 Days Past Due | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	12	85
Commercial | Financing Receivables, 30 to 59 Days Past Due | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	8
Commercial | Financing Receivables, 30 to 59 Days Past Due | Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1	7
Commercial | Financing Receivables 60 To 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	23	35
Commercial | Financing Receivables 60 To 89 Days Past Due | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	17	3
Commercial | Financing Receivables 60 To 89 Days Past Due | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	6	32
Commercial | Financing Receivables 60 To 89 Days Past Due | Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
Commercial | Financing Receivables 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	54	83
Commercial | Financing Receivables 90 Days or More Past Due | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	51	78
Commercial | Financing Receivables 90 Days or More Past Due | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	2	5
Commercial | Financing Receivables 90 Days or More Past Due | Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1	0
Retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	946	1,039
Retail | Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	181	183
Retail | Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	40	53
Retail | Home equity lines of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	242	282
Retail | Home equity loans serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	21	23
Retail | Home equity lines of credit serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	14	10
Retail | Automobile
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	287	338
Retail | Education
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	50	47
Retail | Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	44	44
Retail | Other retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	67	59
Retail | Financing Receivables, 30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	379	393
Retail | Financing Receivables, 30 to 59 Days Past Due | Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	32	37
Retail | Financing Receivables, 30 to 59 Days Past Due | Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	11	12
Retail | Financing Receivables, 30 to 59 Days Past Due | Home equity lines of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	58	65
Retail | Financing Receivables, 30 to 59 Days Past Due | Home equity loans serviced by others
Financing Receivable, Recorded Investment, 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, Recorded Investment, Past Due [Line Items]
Past Due	2	2
Retail | Financing Receivables, 30 to 59 Days Past Due | Automobile
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	201	207
Retail | Financing Receivables, 30 to 59 Days Past Due | Education
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	25	23
Retail | Financing Receivables, 30 to 59 Days Past Due | Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	15	14
Retail | Financing Receivables, 30 to 59 Days Past Due | Other retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	30	26
Retail | Financing Receivables 60 To 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	165	142
Retail | Financing Receivables 60 To 89 Days Past Due | Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	15	13
Retail | Financing Receivables 60 To 89 Days Past Due | Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	5	3
Retail | Financing Receivables 60 To 89 Days Past Due | Home equity lines of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	31	22
Retail | Financing Receivables 60 To 89 Days Past Due | Home equity loans serviced by others
Financing Receivable, Recorded Investment, 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, Recorded Investment, Past Due [Line Items]
Past Due	1	1
Retail | Financing Receivables 60 To 89 Days Past Due | Automobile
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	67	59
Retail | Financing Receivables 60 To 89 Days Past Due | Education
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	13	13
Retail | Financing Receivables 60 To 89 Days Past Due | Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	9	10
Retail | Financing Receivables 60 To 89 Days Past Due | Other retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	21	18
Retail | Financing Receivables 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	402	504
Retail | Financing Receivables 90 Days or More Past Due | Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	134	133
Retail | Financing Receivables 90 Days or More Past Due | Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	24	38
Retail | Financing Receivables 90 Days or More Past Due | Home equity lines of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	153	195
Retail | Financing Receivables 90 Days or More Past Due | Home equity loans serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	13	13
Retail | Financing Receivables 90 Days or More Past Due | Home equity lines of credit serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	11	7
Retail | Financing Receivables 90 Days or More Past Due | Automobile
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	19	72
Retail | Financing Receivables 90 Days or More Past Due | Education
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	12	11
Retail | Financing Receivables 90 Days or More Past Due | Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	20	20
Retail | Financing Receivables 90 Days or More Past Due | Other retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 16	$ 15