ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Default of Modified Debt Agreements (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) contract	Dec. 31, 2017 USD ($) contract	Dec. 31, 2016 USD ($) contract
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	1,103	1,116	1,074
Balance Defaulted | $	$ 103	$ 50	$ 60
Commercial | Commercial Banking
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	29	9	23
Balance Defaulted | $	$ 63	$ 9	$ 13
Commercial | Commercial
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	28	8	22
Balance Defaulted | $	$ 63	$ 5	$ 13
Commercial | Commercial real estate
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	1	1	1
Balance Defaulted | $	$ 0	$ 4	$ 0
Retail | Retail
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	1,074	1,107	1,051
Balance Defaulted | $	$ 40	$ 41	$ 47
Retail | Residential mortgages
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	128	152	187
Balance Defaulted | $	$ 15	$ 19	$ 24
Retail | Home equity loans
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	31	43	50
Balance Defaulted | $	$ 2	$ 2	$ 3
Retail | Home equity lines of credit
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	238	200	155
Balance Defaulted | $	$ 18	$ 14	$ 13
Retail | Home equity loans serviced by others
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	15	23	37
Balance Defaulted | $	$ 0	$ 0	$ 1
Retail | Home equity lines of credit serviced by others
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	6	10	17
Balance Defaulted | $	$ 0	$ 1	$ 0
Retail | Automobile
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	167	140	110
Balance Defaulted | $	$ 2	$ 1	$ 2
Retail | Education
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	23	44	59
Balance Defaulted | $	$ 1	$ 1	$ 1
Retail | Credit cards
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	465	491	433
Balance Defaulted | $	$ 2	$ 3	$ 3
Retail | Other retail
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	1	4	3
Balance Defaulted | $	$ 0	$ 0	$ 0