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, 2019	Dec. 31, 2018
Financing Receivable, Recorded Investment [Line Items]
Total	$ 116,838	$ 116,660
Commercial
Financing Receivable, Recorded Investment [Line Items]
Total	56,963	56,783
Commercial | Pass
Financing Receivable, Recorded Investment [Line Items]
Total	54,294	53,946
Commercial | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Total	1,597	1,682
Commercial | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total	851	951
Commercial | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total	221	204
Commercial | Commercial
Financing Receivable, Recorded Investment [Line Items]
Total	41,156	40,857
Commercial | Commercial | Pass
Financing Receivable, Recorded Investment [Line Items]
Total	39,011	38,600
Commercial | Commercial | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Total	1,174	1,231
Commercial | Commercial | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total	770	828
Commercial | Commercial | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total	201	198
Commercial | Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total	13,123	13,023
Commercial | Commercial real estate | Pass
Financing Receivable, Recorded Investment [Line Items]
Total	12,705	12,523
Commercial | Commercial real estate | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Total	377	412
Commercial | Commercial real estate | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total	38	82
Commercial | Commercial real estate | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total	3	6
Commercial | Leases
Financing Receivable, Recorded Investment [Line Items]
Total	2,684	2,903
Commercial | Leases | Pass
Financing Receivable, Recorded Investment [Line Items]
Total	2,578	2,823
Commercial | Leases | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Total	46	39
Commercial | Leases | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total	43	41
Commercial | Leases | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total	$ 17	$ 0