ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Impaired Loans by Class (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Impaired Loans With a Related Allowance	$ 483	$ 506
Allowance on Impaired Loans	64	81
Impaired Loans Without a Related Allowance	631	625
Unpaid Contractual Balance	1,363	1,364
Total Recorded Investment in Impaired Loans	1,114	1,131
Commercial | Commercial Banking
Financing Receivable, Recorded Investment, Past Due [Line Items]
Impaired Loans With a Related Allowance	218	208
Allowance on Impaired Loans	38	47
Impaired Loans Without a Related Allowance	173	162
Unpaid Contractual Balance	488	443
Total Recorded Investment in Impaired Loans	391	370
Commercial | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Impaired Loans With a Related Allowance	186	183
Allowance on Impaired Loans	31	42
Impaired Loans Without a Related Allowance	167	159
Unpaid Contractual Balance	450	403
Total Recorded Investment in Impaired Loans	353	342
Commercial | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Impaired Loans With a Related Allowance	32	25
Allowance on Impaired Loans	7	5
Impaired Loans Without a Related Allowance	6	3
Unpaid Contractual Balance	38	40
Total Recorded Investment in Impaired Loans	38	28
Commercial | Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Impaired Loans With a Related Allowance	0	0
Allowance on Impaired Loans	0	0
Impaired Loans Without a Related Allowance	0	0
Unpaid Contractual Balance	0	0
Total Recorded Investment in Impaired Loans	0	0
Retail | Retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Impaired Loans With a Related Allowance	265	298
Allowance on Impaired Loans	26	34
Impaired Loans Without a Related Allowance	458	463
Unpaid Contractual Balance	875	921
Total Recorded Investment in Impaired Loans	723	761
Retail | Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Impaired Loans With a Related Allowance	28	25
Allowance on Impaired Loans	2	2
Impaired Loans Without a Related Allowance	127	126
Unpaid Contractual Balance	201	197
Total Recorded Investment in Impaired Loans	155	151
Retail | Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Impaired Loans With a Related Allowance	34	41
Allowance on Impaired Loans	3	4
Impaired Loans Without a Related Allowance	76	80
Unpaid Contractual Balance	148	162
Total Recorded Investment in Impaired Loans	110	121
Retail | Home equity lines of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Impaired Loans With a Related Allowance	21	16
Allowance on Impaired Loans	1	1
Impaired Loans Without a Related Allowance	181	181
Unpaid Contractual Balance	244	241
Total Recorded Investment in Impaired Loans	202	197
Retail | Home equity loans serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Impaired Loans With a Related Allowance	22	29
Allowance on Impaired Loans	1	2
Impaired Loans Without a Related Allowance	19	22
Unpaid Contractual Balance	54	67
Total Recorded Investment in Impaired Loans	41	51
Retail | Home equity lines of credit serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Impaired Loans With a Related Allowance	1	2
Allowance on Impaired Loans	0	0
Impaired Loans Without a Related Allowance	7	7
Unpaid Contractual Balance	11	14
Total Recorded Investment in Impaired Loans	8	9
Retail | Automobile
Financing Receivable, Recorded Investment, Past Due [Line Items]
Impaired Loans With a Related Allowance	1	2
Allowance on Impaired Loans	0	0
Impaired Loans Without a Related Allowance	22	21
Unpaid Contractual Balance	31	30
Total Recorded Investment in Impaired Loans	23	23
Retail | Education
Financing Receivable, Recorded Investment, Past Due [Line Items]
Impaired Loans With a Related Allowance	130	154
Allowance on Impaired Loans	11	17
Impaired Loans Without a Related Allowance	23	21
Unpaid Contractual Balance	153	175
Total Recorded Investment in Impaired Loans	153	175
Retail | Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Impaired Loans With a Related Allowance	24	24
Allowance on Impaired Loans	7	7
Impaired Loans Without a Related Allowance	1	1
Unpaid Contractual Balance	25	25
Total Recorded Investment in Impaired Loans	25	25
Retail | Other retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Impaired Loans With a Related Allowance	4	5
Allowance on Impaired Loans	1	1
Impaired Loans Without a Related Allowance	2	4
Unpaid Contractual Balance	8	10
Total Recorded Investment in Impaired Loans	$ 6	$ 9