ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Accruing and Nonaccruing Past Due Amounts (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Financing Receivable, Past Due [Line Items]
Current	$ 124,328	$ 117,946
Total	125,713	119,088
Financing Receivables, 30 to 59 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	511	511
Financing Receivables 60 To 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	271	212
Financing Receivables 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Past Due	603	419
Commercial
Financing Receivable, Past Due [Line Items]
Current	64,455	57,358
Total	64,930	57,538
Commercial | Commercial
Financing Receivable, Past Due [Line Items]
Current	47,768	41,340
Total	48,017	41,479
Commercial | Commercial real estate
Financing Receivable, Past Due [Line Items]
Current	14,340	13,520
Total	14,485	13,522
Commercial | Leases
Financing Receivable, Past Due [Line Items]
Current	2,347	2,498
Total	2,428	2,537
Commercial | Financing Receivables, 30 to 59 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	157	83
Commercial | Financing Receivables, 30 to 59 Days Past Due | Commercial
Financing Receivable, Past Due [Line Items]
Past Due	60	45
Commercial | Financing Receivables, 30 to 59 Days Past Due | Commercial real estate
Financing Receivable, Past Due [Line Items]
Past Due	92	1
Commercial | Financing Receivables, 30 to 59 Days Past Due | Leases
Financing Receivable, Past Due [Line Items]
Past Due	5	37
Commercial | Financing Receivables 60 To 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	94	28
Commercial | Financing Receivables 60 To 89 Days Past Due | Commercial
Financing Receivable, Past Due [Line Items]
Past Due	30	27
Commercial | Financing Receivables 60 To 89 Days Past Due | Commercial real estate
Financing Receivable, Past Due [Line Items]
Past Due	52	1
Commercial | Financing Receivables 60 To 89 Days Past Due | Leases
Financing Receivable, Past Due [Line Items]
Past Due	12	0
Commercial | Financing Receivables 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Past Due	224	69
Commercial | Financing Receivables 90 Days or More Past Due | Commercial
Financing Receivable, Past Due [Line Items]
Past Due	159	67
Commercial | Financing Receivables 90 Days or More Past Due | Commercial real estate
Financing Receivable, Past Due [Line Items]
Past Due	1	0
Commercial | Financing Receivables 90 Days or More Past Due | Leases
Financing Receivable, Past Due [Line Items]
Past Due	64	2
Retail
Financing Receivable, Past Due [Line Items]
Current	59,873	60,588
Total	60,783	61,550
Retail | Residential mortgages
Financing Receivable, Past Due [Line Items]
Current	19,044	18,947
Total	19,245	19,083
Retail | Home equity
Financing Receivable, Past Due [Line Items]
Current	12,212	12,834
Total	12,541	13,154
Retail | Automobile
Financing Receivable, Past Due [Line Items]
Current	11,784	11,788
Total	12,028	12,120
Retail | Education
Financing Receivable, Past Due [Line Items]
Current	10,548	10,290
Total	10,591	10,347
Retail | Other retail
Financing Receivable, Past Due [Line Items]
Current	6,285	6,729
Total	6,378	6,846
Retail | Financing Receivables, 30 to 59 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	354	428
Retail | Financing Receivables, 30 to 59 Days Past Due | Residential mortgages
Financing Receivable, Past Due [Line Items]
Past Due	65	35
Retail | Financing Receivables, 30 to 59 Days Past Due | Home equity
Financing Receivable, Past Due [Line Items]
Past Due	71	91
Retail | Financing Receivables, 30 to 59 Days Past Due | Automobile
Financing Receivable, Past Due [Line Items]
Past Due	162	227
Retail | Financing Receivables, 30 to 59 Days Past Due | Education
Financing Receivable, Past Due [Line Items]
Past Due	25	30
Retail | Financing Receivables, 30 to 59 Days Past Due | Other retail
Financing Receivable, Past Due [Line Items]
Past Due	31	45
Retail | Financing Receivables 60 To 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	177	184
Retail | Financing Receivables 60 To 89 Days Past Due | Residential mortgages
Financing Receivable, Past Due [Line Items]
Past Due	33	17
Retail | Financing Receivables 60 To 89 Days Past Due | Home equity
Financing Receivable, Past Due [Line Items]
Past Due	41	40
Retail | Financing Receivables 60 To 89 Days Past Due | Automobile
Financing Receivable, Past Due [Line Items]
Past Due	66	81
Retail | Financing Receivables 60 To 89 Days Past Due | Education
Financing Receivable, Past Due [Line Items]
Past Due	12	15
Retail | Financing Receivables 60 To 89 Days Past Due | Other retail
Financing Receivable, Past Due [Line Items]
Past Due	25	31
Retail | Financing Receivables 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Past Due	379	350
Retail | Financing Receivables 90 Days or More Past Due | Residential mortgages
Financing Receivable, Past Due [Line Items]
Past Due	103	84
Retail | Financing Receivables 90 Days or More Past Due | Home equity
Financing Receivable, Past Due [Line Items]
Past Due	217	189
Retail | Financing Receivables 90 Days or More Past Due | Automobile
Financing Receivable, Past Due [Line Items]
Past Due	16	24
Retail | Financing Receivables 90 Days or More Past Due | Education
Financing Receivable, Past Due [Line Items]
Past Due	6	12
Retail | Financing Receivables 90 Days or More Past Due | Other retail
Financing Receivable, Past Due [Line Items]
Past Due	$ 37	$ 41