ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Accruing and Nonaccruing Past Due Amounts (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 1,257	$ 1,456
Financing Receivables, 30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	493	501
Financing Receivables, 60 to 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	177	163
Financing Receivables, 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	587	792
Commercial | Commercial Banking
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	218	366
Commercial | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	166	273
Commercial | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	45	88
Commercial | Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	7	5
Commercial | Financing Receivables, 30 to 59 Days Past Due | Commercial Banking
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	100	68
Commercial | Financing Receivables, 30 to 59 Days Past Due | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	85	26
Commercial | Financing Receivables, 30 to 59 Days Past Due | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	8	38
Commercial | Financing Receivables, 30 to 59 Days Past Due | Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	7	4
Commercial | Financing Receivables, 60 to 89 Days Past Due | Commercial Banking
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	35	25
Commercial | Financing Receivables, 60 to 89 Days Past Due | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	3	4
Commercial | Financing Receivables, 60 to 89 Days Past Due | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	32	20
Commercial | Financing Receivables, 60 to 89 Days Past Due | Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	1
Commercial | Financing Receivables, 90 Days or More Past Due | Commercial Banking
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	83	273
Commercial | Financing Receivables, 90 Days or More Past Due | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	78	243
Commercial | Financing Receivables, 90 Days or More Past Due | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	5	30
Commercial | Financing Receivables, 90 Days or More Past Due | Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
Retail | Retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,039	1,090
Retail | Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	183	184
Retail | Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	53	78
Retail | Home equity lines of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	282	289
Retail | Home equity loans serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	23	36
Retail | Home equity lines of credit serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	10	12
Retail | Automobile
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	338	335
Retail | Education
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	47	76
Retail | Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	44	38
Retail | Other retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	59	42
Retail | Financing Receivables, 30 to 59 Days Past Due | Retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	393	433
Retail | Financing Receivables, 30 to 59 Days Past Due | Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	37	46
Retail | Financing Receivables, 30 to 59 Days Past Due | Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	12	20
Retail | Financing Receivables, 30 to 59 Days Past Due | Home equity lines of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	65	78
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	7	10
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	4
Retail | Financing Receivables, 30 to 59 Days Past Due | Automobile
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	207	220
Retail | Financing Receivables, 30 to 59 Days Past Due | Education
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	23	23
Retail | Financing Receivables, 30 to 59 Days Past Due | Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	14	12
Retail | Financing Receivables, 30 to 59 Days Past Due | Other retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	26	20
Retail | Financing Receivables, 60 to 89 Days Past Due | Retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	142	138
Retail | Financing Receivables, 60 to 89 Days Past Due | Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	13	18
Retail | Financing Receivables, 60 to 89 Days Past Due | Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	3	4
Retail | Financing Receivables, 60 to 89 Days Past Due | Home equity lines of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	22	23
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	4
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	59	55
Retail | Financing Receivables, 60 to 89 Days Past Due | Education
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	13	12
Retail | Financing Receivables, 60 to 89 Days Past Due | Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	10	9
Retail | Financing Receivables, 60 to 89 Days Past Due | Other retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	18	12
Retail | Financing Receivables, 90 Days or More Past Due | Retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	504	519
Retail | Financing Receivables, 90 Days or More Past Due | Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	133	120
Retail | Financing Receivables, 90 Days or More Past Due | Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	38	54
Retail | Financing Receivables, 90 Days or More Past Due | Home equity lines of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	195	188
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	22
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	7	7
Retail | Financing Receivables, 90 Days or More Past Due | Automobile
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	72	60
Retail | Financing Receivables, 90 Days or More Past Due | Education
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	11	41
Retail | Financing Receivables, 90 Days or More Past Due | Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	20	17
Retail | Financing Receivables, 90 Days or More Past Due | Other retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 15	$ 10