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, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment [Line Items]
Total	$ 110,617	$ 107,669
Commercial
Financing Receivable, Recorded Investment [Line Items]
Total	52,031	51,651
Commercial | Commercial Banking
Financing Receivable, Recorded Investment [Line Items]
Total	52,031	51,651
Commercial | Commercial
Financing Receivable, Recorded Investment [Line Items]
Total	37,562	37,274
Commercial | Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total	11,308	10,624
Commercial | Leases
Financing Receivable, Recorded Investment [Line Items]
Total	3,161	3,753
Commercial | Pass | Commercial Banking
Financing Receivable, Recorded Investment [Line Items]
Total	49,205	48,739
Commercial | Pass | Commercial
Financing Receivable, Recorded Investment [Line Items]
Total	35,430	35,010
Commercial | Pass | Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total	10,706	10,146
Commercial | Pass | Leases
Financing Receivable, Recorded Investment [Line Items]
Total	3,069	3,583
Commercial | Special Mention | Commercial Banking
Financing Receivable, Recorded Investment [Line Items]
Total	1,716	1,437
Commercial | Special Mention | Commercial
Financing Receivable, Recorded Investment [Line Items]
Total	1,143	1,015
Commercial | Special Mention | Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total	500	370
Commercial | Special Mention | Leases
Financing Receivable, Recorded Investment [Line Items]
Total	73	52
Commercial | Substandard | Commercial Banking
Financing Receivable, Recorded Investment [Line Items]
Total	878	1,188
Commercial | Substandard | Commercial
Financing Receivable, Recorded Investment [Line Items]
Total	785	1,027
Commercial | Substandard | Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total	74	58
Commercial | Substandard | Leases
Financing Receivable, Recorded Investment [Line Items]
Total	19	103
Commercial | Doubtful | Commercial Banking
Financing Receivable, Recorded Investment [Line Items]
Total	232	287
Commercial | Doubtful | Commercial
Financing Receivable, Recorded Investment [Line Items]
Total	204	222
Commercial | Doubtful | Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total	28	50
Commercial | Doubtful | Leases
Financing Receivable, Recorded Investment [Line Items]
Total	$ 0	$ 15