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, 2019	Dec. 31, 2018
Financing Receivable, Recorded Investment [Line Items]
Total	$ 117,615	$ 116,660
Commercial
Financing Receivable, Recorded Investment [Line Items]
Total	57,689	56,783
Commercial | Pass
Financing Receivable, Recorded Investment [Line Items]
Total	54,919	53,946
Commercial | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Total	1,588	1,682
Commercial | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total	967	951
Commercial | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total	215	204
Commercial | Commercial
Financing Receivable, Recorded Investment [Line Items]
Total	41,497	40,857
Commercial | Commercial | Pass
Financing Receivable, Recorded Investment [Line Items]
Total	39,296	38,600
Commercial | Commercial | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Total	1,096	1,231
Commercial | Commercial | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total	892	828
Commercial | Commercial | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total	213	198
Commercial | Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total	13,372	13,023
Commercial | Commercial real estate | Pass
Financing Receivable, Recorded Investment [Line Items]
Total	12,953	12,523
Commercial | Commercial real estate | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Total	381	412
Commercial | Commercial real estate | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total	36	82
Commercial | Commercial real estate | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total	2	6
Commercial | Leases
Financing Receivable, Recorded Investment [Line Items]
Total	2,820	2,903
Commercial | Leases | Pass
Financing Receivable, Recorded Investment [Line Items]
Total	2,670	2,823
Commercial | Leases | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Total	111	39
Commercial | Leases | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total	39	41
Commercial | Leases | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total	$ 0	$ 0