ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Recorded Investment in Retail Loans by Delinquency Status (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 1,465	$ 1,356
Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	200	316
Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	103	148
Home equity lines of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	272	285
Home equity loans serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	36	40
Home equity lines of credit serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	36	45
Automobile
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	252	194
Student
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	80	71
Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	37	36
Other retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	16	17
Loans and Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	107,669	99,042
Loans and Leases | Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	14,807	12,905
Total	15,115	13,318
Loans and Leases | Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,628	2,245
Total	1,858	2,557
Loans and Leases | Home equity lines of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	13,432	13,982
Total	14,100	14,674
Loans and Leases | Home equity loans serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	673	886
Total	750	986
Loans and Leases | Home equity lines of credit serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	158	296
Total	219	389
Loans and Leases | Automobile
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	12,509	12,670
Total	13,938	13,828
Loans and Leases | Student
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	6,379	4,175
Total	6,610	4,359
Loans and Leases | Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,611	1,554
Total	1,691	1,634
Loans and Leases | Other retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,676	1,013
Total	1,737	1,083
Loans and Leases | Total retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	52,873	49,726
Total	56,018	52,828
Financing Receivables, 1 to 29 Days Past Due | Loans and Leases | Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	108	97
Financing Receivables, 1 to 29 Days Past Due | Loans and Leases | Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	127	164
Financing Receivables, 1 to 29 Days Past Due | Loans and Leases | Home equity lines of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	396	407
Financing Receivables, 1 to 29 Days Past Due | Loans and Leases | Home equity loans serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	41	60
Financing Receivables, 1 to 29 Days Past Due | Loans and Leases | Home equity lines of credit serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	25	48
Financing Receivables, 1 to 29 Days Past Due | Loans and Leases | Automobile
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,177	964
Financing Receivables, 1 to 29 Days Past Due | Loans and Leases | Student
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	151	113
Financing Receivables, 1 to 29 Days Past Due | Loans and Leases | Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	43	44
Financing Receivables, 1 to 29 Days Past Due | Loans and Leases | Other retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	45	53
Financing Receivables, 1 to 29 Days Past Due | Loans and Leases | Total retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	2,113	1,950
Financing Receivables, 30 to 59 Days Past Due | Loans and Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	404	383
Financing Receivables, 30 to 59 Days Past Due | Loans and Leases | Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	53	54
Financing Receivables, 30 to 59 Days Past Due | Loans and Leases | Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	23	32
Financing Receivables, 30 to 59 Days Past Due | Loans and Leases | Home equity lines of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	57	60
Financing Receivables, 30 to 59 Days Past Due | Loans and Leases | Home equity loans serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	14	14
Financing Receivables, 30 to 59 Days Past Due | Loans and Leases | Home equity lines of credit serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	3	10
Financing Receivables, 30 to 59 Days Past Due | Loans and Leases | Automobile
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	172	127
Financing Receivables, 30 to 59 Days Past Due | Loans and Leases | Student
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	24	19
Financing Receivables, 30 to 59 Days Past Due | Loans and Leases | Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	12	11
Financing Receivables, 30 to 59 Days Past Due | Loans and Leases | Other retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	8	8
Financing Receivables, 30 to 59 Days Past Due | Loans and Leases | Total retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	366	335
Financing Receivables, 60 to 89 Days Past Due | Loans and Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	116	124
Financing Receivables, 60 to 89 Days Past Due | Loans and Leases | Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	12	16
Financing Receivables, 60 to 89 Days Past Due | Loans and Leases | Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	7	12
Financing Receivables, 60 to 89 Days Past Due | Loans and Leases | Home equity lines of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	20	20
Financing Receivables, 60 to 89 Days Past Due | Loans and Leases | Home equity loans serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	5	6
Financing Receivables, 60 to 89 Days Past Due | Loans and Leases | Home equity lines of credit serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	2	6
Financing Receivables, 60 to 89 Days Past Due | Loans and Leases | Automobile
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	38	32
Financing Receivables, 60 to 89 Days Past Due | Loans and Leases | Student
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	13	11
Financing Receivables, 60 to 89 Days Past Due | Loans and Leases | Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	9	9
Financing Receivables, 60 to 89 Days Past Due | Loans and Leases | Other retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	4	4
Financing Receivables, 60 to 89 Days Past Due | Loans and Leases | Total retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	110	116
Financing Receivables, 90 Days or More Past Due | Loans and Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	945	849
Financing Receivables, 90 Days or More Past Due | Loans and Leases | Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	135	246
Financing Receivables, 90 Days or More Past Due | Loans and Leases | Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	73	104
Financing Receivables, 90 Days or More Past Due | Loans and Leases | Home equity lines of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	195	205
Financing Receivables, 90 Days or More Past Due | Loans and Leases | Home equity loans serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	17	20
Financing Receivables, 90 Days or More Past Due | Loans and Leases | Home equity lines of credit serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	31	29
Financing Receivables, 90 Days or More Past Due | Loans and Leases | Automobile
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	42	35
Financing Receivables, 90 Days or More Past Due | Loans and Leases | Student
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	43	41
Financing Receivables, 90 Days or More Past Due | Loans and Leases | Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	16	16
Financing Receivables, 90 Days or More Past Due | Loans and Leases | Other retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	4	5
Financing Receivables, 90 Days or More Past Due | Loans and Leases | Total retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 556	$ 701