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	Jun. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Total	$ 113,407	$ 110,617
Commercial
Financing Receivable, Recorded Investment [Line Items]
Total	54,888	52,031
Commercial | Pass
Financing Receivable, Recorded Investment [Line Items]
Total	51,575	49,205
Commercial | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Total	2,118	1,716
Commercial | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total	912	878
Commercial | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total	283	232
Commercial | Commercial
Financing Receivable, Recorded Investment [Line Items]
Total	39,278	37,562
Commercial | Commercial | Pass
Financing Receivable, Recorded Investment [Line Items]
Total	36,576	35,430
Commercial | Commercial | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Total	1,694	1,143
Commercial | Commercial | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total	754	785
Commercial | Commercial | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total	254	204
Commercial | Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total	12,528	11,308
Commercial | Commercial real estate | Pass
Financing Receivable, Recorded Investment [Line Items]
Total	12,044	10,706
Commercial | Commercial real estate | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Total	336	500
Commercial | Commercial real estate | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total	119	74
Commercial | Commercial real estate | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total	29	28
Commercial | Leases
Financing Receivable, Recorded Investment [Line Items]
Total	3,082	3,161
Commercial | Leases | Pass
Financing Receivable, Recorded Investment [Line Items]
Total	2,955	3,069
Commercial | Leases | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Total	88	73
Commercial | Leases | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total	39	19
Commercial | Leases | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total	$ 0	$ 0