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	Sep. 30, 2019	Dec. 31, 2018
Financing Receivable, Credit Quality Indicator [Line Items]
Total	$ 117,880	$ 116,660
Commercial
Financing Receivable, Credit Quality Indicator [Line Items]
Total	56,733	56,783
Commercial | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total	53,900	53,946
Commercial | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total	1,705	1,682
Commercial | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total	846	951
Commercial | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total	282	204
Commercial | Commercial
Financing Receivable, Credit Quality Indicator [Line Items]
Total	41,356	40,857
Commercial | Commercial | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total	38,997	38,600
Commercial | Commercial | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total	1,369	1,231
Commercial | Commercial | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total	761	828
Commercial | Commercial | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total	229	198
Commercial | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Total	12,820	13,023
Commercial | Commercial real estate | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total	12,437	12,523
Commercial | Commercial real estate | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total	297	412
Commercial | Commercial real estate | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total	36	82
Commercial | Commercial real estate | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total	50	6
Commercial | Leases
Financing Receivable, Credit Quality Indicator [Line Items]
Total	2,557	2,903
Commercial | Leases | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total	2,466	2,823
Commercial | Leases | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total	39	39
Commercial | Leases | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total	49	41
Commercial | Leases | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total	$ 3	$ 0