ALLOWANCE FOR CREDIT LOSSES, NONACCRUING LOANS AND LEASES, AND CONCENTRATIONS OF CREDIT RISK - Accruing and Nonaccruing Past Due Amounts (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Financing Receivable, Past Due [Line Items]
Current	$ 121,212	$ 121,646
Total	122,581	123,090
Financing Receivables, 30 to 59 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	413	594
Financing Receivables 60 To 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	161	257
Financing Receivables 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Past Due	795	593
Commercial
Financing Receivable, Past Due [Line Items]
Current	58,906	60,304
Total	59,083	60,793
Commercial | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Current	42,769	43,817
Total	42,842	44,173
Commercial | Commercial real estate
Financing Receivable, Past Due [Line Items]
Current	14,310	14,531
Total	14,412	14,652
Commercial | Leases
Financing Receivable, Past Due [Line Items]
Current	1,827	1,956
Total	1,829	1,968
Commercial | Financing Receivables, 30 to 59 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	25	233
Commercial | Financing Receivables, 30 to 59 Days Past Due | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Past Due	24	223
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	0	9
Commercial | Financing Receivables 60 To 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	7	101
Commercial | Financing Receivables 60 To 89 Days Past Due | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Past Due	7	16
Commercial | Financing Receivables 60 To 89 Days Past Due | Commercial real estate
Financing Receivable, Past Due [Line Items]
Past Due	0	85
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	145	155
Commercial | Financing Receivables 90 Days or More Past Due | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Past Due	42	117
Commercial | Financing Receivables 90 Days or More Past Due | Commercial real estate
Financing Receivable, Past Due [Line Items]
Past Due	101	35
Commercial | Financing Receivables 90 Days or More Past Due | Leases
Financing Receivable, Past Due [Line Items]
Past Due	2	3
Retail
Financing Receivable, Past Due [Line Items]
Current	62,306	61,342
Total	63,498	62,297
Retail | Residential mortgages (2)
Financing Receivable, Past Due [Line Items]
Current	19,885	19,291
Total	20,538	19,539
Retail | Home equity
Financing Receivable, Past Due [Line Items]
Current	11,607	11,848
Total	11,841	12,149
Retail | Automobile
Financing Receivable, Past Due [Line Items]
Current	12,613	11,901
Total	12,780	12,153
Retail | Education
Financing Receivable, Past Due [Line Items]
Current	12,747	12,255
Total	12,800	12,308
Retail | Other retail
Financing Receivable, Past Due [Line Items]
Current	5,454	6,047
Total	5,539	6,148
Retail | Financing Receivables, 30 to 59 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	388	361
Retail | Financing Receivables, 30 to 59 Days Past Due | Residential mortgages (2)
Financing Receivable, Past Due [Line Items]
Past Due	176	59
Retail | Financing Receivables, 30 to 59 Days Past Due | Home equity
Financing Receivable, Past Due [Line Items]
Past Due	32	61
Retail | Financing Receivables, 30 to 59 Days Past Due | Automobile
Financing Receivable, Past Due [Line Items]
Past Due	114	170
Retail | Financing Receivables, 30 to 59 Days Past Due | Education
Financing Receivable, Past Due [Line Items]
Past Due	29	33
Retail | Financing Receivables, 30 to 59 Days Past Due | Other retail
Financing Receivable, Past Due [Line Items]
Past Due	37	38
Retail | Financing Receivables 60 To 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	154	156
Retail | Financing Receivables 60 To 89 Days Past Due | Residential mortgages (2)
Financing Receivable, Past Due [Line Items]
Past Due	61	21
Retail | Financing Receivables 60 To 89 Days Past Due | Home equity
Financing Receivable, Past Due [Line Items]
Past Due	17	28
Retail | Financing Receivables 60 To 89 Days Past Due | Automobile
Financing Receivable, Past Due [Line Items]
Past Due	43	65
Retail | Financing Receivables 60 To 89 Days Past Due | Education
Financing Receivable, Past Due [Line Items]
Past Due	13	13
Retail | Financing Receivables 60 To 89 Days Past Due | Other retail
Financing Receivable, Past Due [Line Items]
Past Due	20	29
Retail | Financing Receivables 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Past Due	650	438
Retail | Financing Receivables 90 Days or More Past Due | Residential mortgages (2)
Financing Receivable, Past Due [Line Items]
Past Due	416	168
Retail | Financing Receivables 90 Days or More Past Due | Home equity
Financing Receivable, Past Due [Line Items]
Past Due	185	212
Retail | Financing Receivables 90 Days or More Past Due | Automobile
Financing Receivable, Past Due [Line Items]
Past Due	10	17
Retail | Financing Receivables 90 Days or More Past Due | Education
Financing Receivable, Past Due [Line Items]
Past Due	11	7
Retail | Financing Receivables 90 Days or More Past Due | Other retail
Financing Receivable, Past Due [Line Items]
Past Due	$ 28	$ 34