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, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Total	$ 116,660	$ 110,617
Commercial
Financing Receivable, Recorded Investment [Line Items]
Total	56,783	52,031
Commercial | Commercial Banking
Financing Receivable, Recorded Investment [Line Items]
Total	56,783	52,031
Commercial | Commercial
Financing Receivable, Recorded Investment [Line Items]
Total	40,857	37,562
Commercial | Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total	13,023	11,308
Commercial | Leases
Financing Receivable, Recorded Investment [Line Items]
Total	2,903	3,161
Commercial | Pass | Commercial Banking
Financing Receivable, Recorded Investment [Line Items]
Total	53,946	49,205
Commercial | Pass | Commercial
Financing Receivable, Recorded Investment [Line Items]
Total	38,600	35,430
Commercial | Pass | Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total	12,523	10,706
Commercial | Pass | Leases
Financing Receivable, Recorded Investment [Line Items]
Total	2,823	3,069
Commercial | Special Mention | Commercial Banking
Financing Receivable, Recorded Investment [Line Items]
Total	1,682	1,716
Commercial | Special Mention | Commercial
Financing Receivable, Recorded Investment [Line Items]
Total	1,231	1,143
Commercial | Special Mention | Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total	412	500
Commercial | Special Mention | Leases
Financing Receivable, Recorded Investment [Line Items]
Total	39	73
Commercial | Substandard | Commercial Banking
Financing Receivable, Recorded Investment [Line Items]
Total	951	878
Commercial | Substandard | Commercial
Financing Receivable, Recorded Investment [Line Items]
Total	828	785
Commercial | Substandard | Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total	82	74
Commercial | Substandard | Leases
Financing Receivable, Recorded Investment [Line Items]
Total	41	19
Commercial | Doubtful | Commercial Banking
Financing Receivable, Recorded Investment [Line Items]
Total	204	232
Commercial | Doubtful | Commercial
Financing Receivable, Recorded Investment [Line Items]
Total	198	204
Commercial | Doubtful | Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total	6	28
Commercial | Doubtful | Leases
Financing Receivable, Recorded Investment [Line Items]
Total	$ 0	$ 0