ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Recorded Investment in Commercial Loans and Leases by Regulatory Classification Ratings (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable, Credit Quality Indicator [Line Items]
Total	$ 119,088	$ 116,660
Commercial
Financing Receivable, Credit Quality Indicator [Line Items]
Total	57,538	56,783
Commercial | Commercial
Financing Receivable, Credit Quality Indicator [Line Items]
Total	41,479	40,857
Commercial | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Total	13,522	13,023
Commercial | Leases
Financing Receivable, Credit Quality Indicator [Line Items]
Total	2,537	2,903
Commercial | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total	54,502	53,946
Commercial | Pass | Commercial
Financing Receivable, Credit Quality Indicator [Line Items]
Total	38,950	38,600
Commercial | Pass | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Total	13,169	12,523
Commercial | Pass | Leases
Financing Receivable, Credit Quality Indicator [Line Items]
Total	2,383	2,823
Commercial | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total	1,778	1,682
Commercial | Special Mention | Commercial
Financing Receivable, Credit Quality Indicator [Line Items]
Total	1,351	1,231
Commercial | Special Mention | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Total	318	412
Commercial | Special Mention | Leases
Financing Receivable, Credit Quality Indicator [Line Items]
Total	109	39
Commercial | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total	1,009	951
Commercial | Substandard | Commercial
Financing Receivable, Credit Quality Indicator [Line Items]
Total	934	828
Commercial | Substandard | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Total	33	82
Commercial | Substandard | Leases
Financing Receivable, Credit Quality Indicator [Line Items]
Total	42	41
Commercial | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total	249	204
Commercial | Doubtful | Commercial
Financing Receivable, Credit Quality Indicator [Line Items]
Total	244	198
Commercial | Doubtful | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Total	2	6
Commercial | Doubtful | Leases
Financing Receivable, Credit Quality Indicator [Line Items]
Total	$ 3	$ 0