ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Default of Modified Debt Agreements (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) contract	Dec. 31, 2016 USD ($) contract	Dec. 31, 2015 USD ($) contract
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	1,116	1,074	1,288
Balance Defaulted | $	$ 50	$ 60	$ 52
Commercial | Commercial Banking
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	9	23	23
Balance Defaulted | $	$ 9	$ 13	$ 2
Commercial | Commercial
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	8	22	23
Balance Defaulted | $	$ 5	$ 13	$ 2
Commercial | Commercial real estate
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	1	1	0
Balance Defaulted | $	$ 4	$ 0	$ 0
Commercial | Leases
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	0	0	0
Balance Defaulted | $	$ 0	$ 0	$ 0
Retail | Retail
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	1,107	1,051	1,265
Balance Defaulted | $	$ 41	$ 47	$ 50
Retail | Residential mortgages
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	152	187	168
Balance Defaulted | $	$ 19	$ 24	$ 21
Retail | Home equity loans
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	43	50	184
Balance Defaulted | $	$ 2	$ 3	$ 13
Retail | Home equity lines of credit
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	200	155	131
Balance Defaulted | $	$ 14	$ 13	$ 7
Retail | Home equity loans serviced by others
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	23	37	43
Balance Defaulted | $	$ 0	$ 1	$ 1
Retail | Home equity lines of credit serviced by others
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	10	17	22
Balance Defaulted | $	$ 1	$ 0	$ 1
Retail | Automobile
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	140	110	87
Balance Defaulted | $	$ 1	$ 2	$ 1
Retail | Education
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	44	59	171
Balance Defaulted | $	$ 1	$ 1	$ 3
Retail | Credit cards
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	491	433	455
Balance Defaulted | $	$ 3	$ 3	$ 3
Retail | Other retail
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	4	3	4
Balance Defaulted | $	$ 0	$ 0	$ 0