ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Recorded Investment in Retail Loans by Delinquency Status (Details) - Loans and Leases - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 1,383	$ 1,465
Total	109,046	107,669
Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	15,820	14,807
Past Due	163	200
Total	16,082	15,115
Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,430	1,628
Past Due	80	103
Total	1,606	1,858
Home equity lines of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	13,071	13,432
Past Due	259	272
Total	13,696	14,100
Home equity loans serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	582	673
Past Due	30	36
Total	647	750
Home equity lines of credit serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	135	158
Past Due	25	36
Total	182	219
Automobile
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	12,199	12,509
Past Due	259	252
Total	13,449	13,938
Education
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	7,537	6,379
Past Due	66	80
Total	7,720	6,610
Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,636	1,611
Past Due	32	37
Total	1,711	1,691
Other retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,982	1,676
Past Due	28	16
Total	2,065	1,737
Total retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	54,392	52,873
Total	57,158	56,018
Financing Receivables, 1 to 29 Days Past Due | Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	99	108
Financing Receivables, 1 to 29 Days Past Due | Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	96	127
Financing Receivables, 1 to 29 Days Past Due | Home equity lines of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	366	396
Financing Receivables, 1 to 29 Days Past Due | Home equity loans serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	35	41
Financing Receivables, 1 to 29 Days Past Due | Home equity lines of credit serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	22	25
Financing Receivables, 1 to 29 Days Past Due | Automobile
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	991	1,177
Financing Receivables, 1 to 29 Days Past Due | Education
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	117	151
Financing Receivables, 1 to 29 Days Past Due | Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	43	43
Financing Receivables, 1 to 29 Days Past Due | Other retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	55	45
Financing Receivables, 1 to 29 Days Past Due | Total retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,824	2,113
Financing Receivables, 30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	336	404
Financing Receivables, 30 to 59 Days Past Due | Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	35	53
Financing Receivables, 30 to 59 Days Past Due | Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	12	23
Financing Receivables, 30 to 59 Days Past Due | Home equity lines of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	52	57
Financing Receivables, 30 to 59 Days Past Due | Home equity loans serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	9	14
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	3	3
Financing Receivables, 30 to 59 Days Past Due | Automobile
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	172	172
Financing Receivables, 30 to 59 Days Past Due | Education
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	18	24
Financing Receivables, 30 to 59 Days Past Due | Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	10	12
Financing Receivables, 30 to 59 Days Past Due | Other retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	13	8
Financing Receivables, 30 to 59 Days Past Due | Total retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	324	366
Financing Receivables 60 To 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	121	116
Financing Receivables 60 To 89 Days Past Due | Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	10	12
Financing Receivables 60 To 89 Days Past Due | Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	5	7
Financing Receivables 60 To 89 Days Past Due | Home equity lines of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	22	20
Financing Receivables 60 To 89 Days Past Due | Home equity loans serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	5	5
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	2
Financing Receivables 60 To 89 Days Past Due | Automobile
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	42	38
Financing Receivables 60 To 89 Days Past Due | Education
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	10	13
Financing Receivables 60 To 89 Days Past Due | Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	7	9
Financing Receivables 60 To 89 Days Past Due | Other retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	8	4
Financing Receivables 60 To 89 Days Past Due | Total retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	110	110
Financing Receivables 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	926	945
Financing Receivables 90 Days or More Past Due | Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	118	135
Financing Receivables 90 Days or More Past Due | Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	63	73
Financing Receivables 90 Days or More Past Due | Home equity lines of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	185	195
Financing Receivables 90 Days or More Past Due | Home equity loans serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	16	17
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	21	31
Financing Receivables 90 Days or More Past Due | Automobile
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	45	42
Financing Receivables 90 Days or More Past Due | Education
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	38	43
Financing Receivables 90 Days or More Past Due | Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	15	16
Financing Receivables 90 Days or More Past Due | Other retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	7	4
Financing Receivables 90 Days or More Past Due | Total retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 508	$ 556