ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Accruing and Nonaccruing Past Due Amounts (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Financing Receivable, Past Due [Line Items]
Current	$ 121,646	$ 117,946
Total	123,090	119,088
Financing Receivables, 30 to 59 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	594	511
Financing Receivables, 60 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	257	212
Financing Receivables, 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Past Due	593	419
Commercial
Financing Receivable, Past Due [Line Items]
Current	60,304	57,358
Total	60,793	57,538
Commercial \| Commercial and industrial
Financing Receivable, Past Due [Line Items]
Current	43,817	41,340
Total	44,173	41,479
Commercial \| Commercial real estate
Financing Receivable, Past Due [Line Items]
Current	14,531	13,520
Total	14,652	13,522
Commercial \| Leases
Financing Receivable, Past Due [Line Items]
Current	1,956	2,498
Total	1,968	2,537
Commercial \| Financing Receivables, 30 to 59 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	233	83
Commercial \| Financing Receivables, 30 to 59 Days Past Due \| Commercial and industrial
Financing Receivable, Past Due [Line Items]
Past Due	223	45
Commercial \| Financing Receivables, 30 to 59 Days Past Due \| Commercial real estate
Financing Receivable, Past Due [Line Items]
Past Due	1	1
Commercial \| Financing Receivables, 30 to 59 Days Past Due \| Leases
Financing Receivable, Past Due [Line Items]
Past Due	9	37
Commercial \| Financing Receivables, 60 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	101	28
Commercial \| Financing Receivables, 60 to 89 Days Past Due \| Commercial and industrial
Financing Receivable, Past Due [Line Items]
Past Due	16	27
Commercial \| Financing Receivables, 60 to 89 Days Past Due \| Commercial real estate
Financing Receivable, Past Due [Line Items]
Past Due	85	1
Commercial \| Financing Receivables, 60 to 89 Days Past Due \| Leases
Financing Receivable, Past Due [Line Items]
Past Due	0	0
Commercial \| Financing Receivables, 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Past Due	155	69
Commercial \| Financing Receivables, 90 Days or More Past Due \| Commercial and industrial
Financing Receivable, Past Due [Line Items]
Past Due	117	67
Commercial \| Financing Receivables, 90 Days or More Past Due \| Commercial real estate
Financing Receivable, Past Due [Line Items]
Past Due	35	0
Commercial \| Financing Receivables, 90 Days or More Past Due \| Leases
Financing Receivable, Past Due [Line Items]
Past Due	3	2
Retail
Financing Receivable, Past Due [Line Items]
Current	61,342	60,588
Total	62,297	61,550
Retail \| Residential mortgages
Financing Receivable, Past Due [Line Items]
Current	19,291	18,947
Total	19,539	19,083
Retail \| Home equity
Financing Receivable, Past Due [Line Items]
Current	11,848	12,834
Total	12,149	13,154
Retail \| Automobile
Financing Receivable, Past Due [Line Items]
Current	11,901	11,788
Total	12,153	12,120
Retail \| Education
Financing Receivable, Past Due [Line Items]
Current	12,255	10,290
Total	12,308	10,347
Retail \| Other retail
Financing Receivable, Past Due [Line Items]
Current	6,047	6,729
Total	6,148	6,846
Retail \| Financing Receivables, 30 to 59 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	361	428
Retail \| Financing Receivables, 30 to 59 Days Past Due \| Residential mortgages
Financing Receivable, Past Due [Line Items]
Past Due	59	35
Retail \| Financing Receivables, 30 to 59 Days Past Due \| Home equity
Financing Receivable, Past Due [Line Items]
Past Due	61	91
Retail \| Financing Receivables, 30 to 59 Days Past Due \| Automobile
Financing Receivable, Past Due [Line Items]
Past Due	170	227
Retail \| Financing Receivables, 30 to 59 Days Past Due \| Education
Financing Receivable, Past Due [Line Items]
Past Due	33	30
Retail \| Financing Receivables, 30 to 59 Days Past Due \| Other retail
Financing Receivable, Past Due [Line Items]
Past Due	38	45
Retail \| Financing Receivables, 60 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	156	184
Retail \| Financing Receivables, 60 to 89 Days Past Due \| Residential mortgages
Financing Receivable, Past Due [Line Items]
Past Due	21	17
Retail \| Financing Receivables, 60 to 89 Days Past Due \| Home equity
Financing Receivable, Past Due [Line Items]
Past Due	28	40
Retail \| Financing Receivables, 60 to 89 Days Past Due \| Automobile
Financing Receivable, Past Due [Line Items]
Past Due	65	81
Retail \| Financing Receivables, 60 to 89 Days Past Due \| Education
Financing Receivable, Past Due [Line Items]
Past Due	13	15
Retail \| Financing Receivables, 60 to 89 Days Past Due \| Other retail
Financing Receivable, Past Due [Line Items]
Past Due	29	31
Retail \| Financing Receivables, 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Past Due	438	350
Retail \| Financing Receivables, 90 Days or More Past Due \| Residential mortgages
Financing Receivable, Past Due [Line Items]
Past Due	168	84
Retail \| Financing Receivables, 90 Days or More Past Due \| Home equity
Financing Receivable, Past Due [Line Items]
Past Due	212	189
Retail \| Financing Receivables, 90 Days or More Past Due \| Automobile
Financing Receivable, Past Due [Line Items]
Past Due	17	24
Retail \| Financing Receivables, 90 Days or More Past Due \| Education
Financing Receivable, Past Due [Line Items]
Past Due	7	12
Retail \| Financing Receivables, 90 Days or More Past Due \| Other retail
Financing Receivable, Past Due [Line Items]
Past Due	$ 34	$ 41