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, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Total	$ 114,720	$ 110,617
Commercial
Financing Receivable, Recorded Investment [Line Items]
Total	55,405	52,031
Commercial | Pass
Financing Receivable, Recorded Investment [Line Items]
Total	52,344	49,205
Commercial | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Total	1,772	1,716
Commercial | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total	1,028	878
Commercial | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total	261	232
Commercial | Commercial
Financing Receivable, Recorded Investment [Line Items]
Total	39,770	37,562
Commercial | Commercial | Pass
Financing Receivable, Recorded Investment [Line Items]
Total	37,242	35,430
Commercial | Commercial | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Total	1,441	1,143
Commercial | Commercial | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total	854	785
Commercial | Commercial | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total	233	204
Commercial | Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total	12,630	11,308
Commercial | Commercial real estate | Pass
Financing Receivable, Recorded Investment [Line Items]
Total	12,193	10,706
Commercial | Commercial real estate | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Total	282	500
Commercial | Commercial real estate | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total	127	74
Commercial | Commercial real estate | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total	28	28
Commercial | Leases
Financing Receivable, Recorded Investment [Line Items]
Total	3,005	3,161
Commercial | Leases | Pass
Financing Receivable, Recorded Investment [Line Items]
Total	2,909	3,069
Commercial | Leases | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Total	49	73
Commercial | Leases | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total	47	19
Commercial | Leases | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total	$ 0	$ 0