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	Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Total	$ 111,425	$ 110,617
Commercial
Financing Receivable, Recorded Investment [Line Items]
Total	53,144	52,031
Commercial | Pass
Financing Receivable, Recorded Investment [Line Items]
Total	49,678	49,205
Commercial | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Total	2,226	1,716
Commercial | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total	1,000	878
Commercial | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total	240	232
Commercial | Commercial
Financing Receivable, Recorded Investment [Line Items]
Total	38,277	37,562
Commercial | Commercial | Pass
Financing Receivable, Recorded Investment [Line Items]
Total	35,511	35,430
Commercial | Commercial | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Total	1,644	1,143
Commercial | Commercial | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total	910	785
Commercial | Commercial | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total	212	204
Commercial | Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total	11,775	11,308
Commercial | Commercial real estate | Pass
Financing Receivable, Recorded Investment [Line Items]
Total	11,195	10,706
Commercial | Commercial real estate | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Total	505	500
Commercial | Commercial real estate | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total	47	74
Commercial | Commercial real estate | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total	28	28
Commercial | Leases
Financing Receivable, Recorded Investment [Line Items]
Total	3,092	3,161
Commercial | Leases | Pass
Financing Receivable, Recorded Investment [Line Items]
Total	2,972	3,069
Commercial | Leases | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Total	77	73
Commercial | Leases | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total	43	19
Commercial | Leases | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total	$ 0	$ 0