ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Impaired Loans by Class (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment, Past Due [Line Items]
Impaired Loans With a Related Allowance	$ 506	$ 634
Allowance on Impaired Loans	81	106
Impaired Loans Without a Related Allowance	625	589
Unpaid Contractual Balance	1,364	1,443
Total Recorded Investment in Impaired Loans	1,131	1,223
Commercial | Commercial Banking
Financing Receivable, Recorded Investment, Past Due [Line Items]
Impaired Loans With a Related Allowance	208	286
Allowance on Impaired Loans	47	63
Impaired Loans Without a Related Allowance	162	138
Unpaid Contractual Balance	443	475
Total Recorded Investment in Impaired Loans	370	424
Commercial | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Impaired Loans With a Related Allowance	183	247
Allowance on Impaired Loans	42	55
Impaired Loans Without a Related Allowance	159	134
Unpaid Contractual Balance	403	431
Total Recorded Investment in Impaired Loans	342	381
Commercial | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Impaired Loans With a Related Allowance	25	39
Allowance on Impaired Loans	5	8
Impaired Loans Without a Related Allowance	3	4
Unpaid Contractual Balance	40	44
Total Recorded Investment in Impaired Loans	28	43
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	298	348
Allowance on Impaired Loans	34	43
Impaired Loans Without a Related Allowance	463	451
Unpaid Contractual Balance	921	968
Total Recorded Investment in Impaired Loans	761	799
Retail | Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Impaired Loans With a Related Allowance	25	37
Allowance on Impaired Loans	2	2
Impaired Loans Without a Related Allowance	126	141
Unpaid Contractual Balance	197	235
Total Recorded Investment in Impaired Loans	151	178
Retail | Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Impaired Loans With a Related Allowance	41	51
Allowance on Impaired Loans	4	3
Impaired Loans Without a Related Allowance	80	94
Unpaid Contractual Balance	162	191
Total Recorded Investment in Impaired Loans	121	145
Retail | Home equity lines of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Impaired Loans With a Related Allowance	16	23
Allowance on Impaired Loans	1	1
Impaired Loans Without a Related Allowance	181	173
Unpaid Contractual Balance	241	240
Total Recorded Investment in Impaired Loans	197	196
Retail | Home equity loans serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Impaired Loans With a Related Allowance	29	41
Allowance on Impaired Loans	2	4
Impaired Loans Without a Related Allowance	22	19
Unpaid Contractual Balance	67	70
Total Recorded Investment in Impaired Loans	51	60
Retail | Home equity lines of credit serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Impaired Loans With a Related Allowance	2	2
Allowance on Impaired Loans	0	0
Impaired Loans Without a Related Allowance	7	7
Unpaid Contractual Balance	14	13
Total Recorded Investment in Impaired Loans	9	9
Retail | Automobile
Financing Receivable, Recorded Investment, Past Due [Line Items]
Impaired Loans With a Related Allowance	2	4
Allowance on Impaired Loans	0	0
Impaired Loans Without a Related Allowance	21	15
Unpaid Contractual Balance	30	25
Total Recorded Investment in Impaired Loans	23	19
Retail | Education
Financing Receivable, Recorded Investment, Past Due [Line Items]
Impaired Loans With a Related Allowance	154	154
Allowance on Impaired Loans	17	25
Impaired Loans Without a Related Allowance	21	1
Unpaid Contractual Balance	175	155
Total Recorded Investment in Impaired Loans	175	155
Retail | Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Impaired Loans With a Related Allowance	24	26
Allowance on Impaired Loans	7	6
Impaired Loans Without a Related Allowance	1	0
Unpaid Contractual Balance	25	26
Total Recorded Investment in Impaired Loans	25	26
Retail | Other retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Impaired Loans With a Related Allowance	5	10
Allowance on Impaired Loans	1	2
Impaired Loans Without a Related Allowance	4	1
Unpaid Contractual Balance	10	13
Total Recorded Investment in Impaired Loans	$ 9	$ 11