ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Accruing and Nonaccruing Past Due Amounts (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Financing Receivable, Past Due [Line Items]
Current	$ 122,724	$ 117,946
Total	124,071	119,088
Financing Receivables, 30 to 59 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	519	511
Financing Receivables 60 To 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	261	212
Financing Receivables 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Past Due	567	419
Commercial
Financing Receivable, Past Due [Line Items]
Current	61,872	57,358
Total	62,362	57,538
Commercial | Commercial
Financing Receivable, Past Due [Line Items]
Current	44,845	41,340
Total	45,185	41,479
Commercial | Commercial real estate
Financing Receivable, Past Due [Line Items]
Current	14,743	13,520
Total	14,889	13,522
Commercial | Leases
Financing Receivable, Past Due [Line Items]
Current	2,284	2,498
Total	2,288	2,537
Commercial | Financing Receivables, 30 to 59 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	196	83
Commercial | Financing Receivables, 30 to 59 Days Past Due | Commercial
Financing Receivable, Past Due [Line Items]
Past Due	105	45
Commercial | Financing Receivables, 30 to 59 Days Past Due | Commercial real estate
Financing Receivable, Past Due [Line Items]
Past Due	90	1
Commercial | Financing Receivables, 30 to 59 Days Past Due | Leases
Financing Receivable, Past Due [Line Items]
Past Due	1	37
Commercial | Financing Receivables 60 To 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	130	28
Commercial | Financing Receivables 60 To 89 Days Past Due | Commercial
Financing Receivable, Past Due [Line Items]
Past Due	129	27
Commercial | Financing Receivables 60 To 89 Days Past Due | Commercial real estate
Financing Receivable, Past Due [Line Items]
Past Due	0	1
Commercial | Financing Receivables 60 To 89 Days Past Due | Leases
Financing Receivable, Past Due [Line Items]
Past Due	1	0
Commercial | Financing Receivables 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Past Due	164	69
Commercial | Financing Receivables 90 Days or More Past Due | Commercial
Financing Receivable, Past Due [Line Items]
Past Due	106	67
Commercial | Financing Receivables 90 Days or More Past Due | Commercial real estate
Financing Receivable, Past Due [Line Items]
Past Due	56	0
Commercial | Financing Receivables 90 Days or More Past Due | Leases
Financing Receivable, Past Due [Line Items]
Past Due	2	2
Retail
Financing Receivable, Past Due [Line Items]
Current	60,852	60,588
Total	61,709	61,550
Retail | Residential mortgages
Financing Receivable, Past Due [Line Items]
Current	19,430	18,947
Total	19,633	19,083
Retail | Home equity
Financing Receivable, Past Due [Line Items]
Current	12,007	12,834
Total	12,322	13,154
Retail | Automobile
Financing Receivable, Past Due [Line Items]
Current	11,825	11,788
Total	12,035	12,120
Retail | Education
Financing Receivable, Past Due [Line Items]
Current	11,585	10,290
Total	11,631	10,347
Retail | Other retail
Financing Receivable, Past Due [Line Items]
Current	6,005	6,729
Total	6,088	6,846
Retail | Financing Receivables, 30 to 59 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	323	428
Retail | Financing Receivables, 30 to 59 Days Past Due | Residential mortgages
Financing Receivable, Past Due [Line Items]
Past Due	64	35
Retail | Financing Receivables, 30 to 59 Days Past Due | Home equity
Financing Receivable, Past Due [Line Items]
Past Due	51	91
Retail | Financing Receivables, 30 to 59 Days Past Due | Automobile
Financing Receivable, Past Due [Line Items]
Past Due	147	227
Retail | Financing Receivables, 30 to 59 Days Past Due | Education
Financing Receivable, Past Due [Line Items]
Past Due	28	30
Retail | Financing Receivables, 30 to 59 Days Past Due | Other retail
Financing Receivable, Past Due [Line Items]
Past Due	33	45
Retail | Financing Receivables 60 To 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	131	184
Retail | Financing Receivables 60 To 89 Days Past Due | Residential mortgages
Financing Receivable, Past Due [Line Items]
Past Due	16	17
Retail | Financing Receivables 60 To 89 Days Past Due | Home equity
Financing Receivable, Past Due [Line Items]
Past Due	29	40
Retail | Financing Receivables 60 To 89 Days Past Due | Automobile
Financing Receivable, Past Due [Line Items]
Past Due	52	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	22	31
Retail | Financing Receivables 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Past Due	403	350
Retail | Financing Receivables 90 Days or More Past Due | Residential mortgages
Financing Receivable, Past Due [Line Items]
Past Due	123	84
Retail | Financing Receivables 90 Days or More Past Due | Home equity
Financing Receivable, Past Due [Line Items]
Past Due	235	189
Retail | Financing Receivables 90 Days or More Past Due | Automobile
Financing Receivable, Past Due [Line Items]
Past Due	11	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	$ 28	$ 41