ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Additional Impaired Loan Information (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	$ 11	$ 7	$ 31	$ 25
Average Recorded Investment	1,097	1,172	1,057	1,160
Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	3	1	7	3
Average Recorded Investment	368	424	351	441
Commercial \| Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	3	1	7	3
Average Recorded Investment	334	391	318	402
Commercial \| Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	0	0	0	0
Average Recorded Investment	34	33	33	39
Commercial \| Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	0	0	0	0
Average Recorded Investment	0	0	0	0
Retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	8	6	24	22
Average Recorded Investment	729	748	706	719
Retail \| Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	1	0	4	3
Average Recorded Investment	154	137	148	128
Retail \| Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	1	1	4	4
Average Recorded Investment	107	125	107	124
Retail \| Home equity lines of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	2	2	6	5
Average Recorded Investment	202	192	189	178
Retail \| Home equity loans serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	1	0	2	2
Average Recorded Investment	43	51	44	51
Retail \| Home equity lines of credit serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	0	0	0	0
Average Recorded Investment	9	9	9	9
Retail \| Automobile
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	0	0	0	0
Average Recorded Investment	23	21	21	18
Retail \| Education
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	3	3	7	7
Average Recorded Investment	160	178	159	178
Retail \| Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	0	0	1	1
Average Recorded Investment	24	25	22	23
Retail \| Other retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	0	0	0	0
Average Recorded Investment	$ 7	$ 10	$ 7	$ 10

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Average amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5212-111524

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5258-111524

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Amount of interest income recognized that the financing receivables (with related allowance for credit losses and without a related allowance for credit losses) were impaired.

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