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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	$ 7	$ 9	$ 25	$ 27
Average Recorded Investment	1,172	1,180	1,160	1,106
Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	1	1	3	4
Average Recorded Investment	424	380	441	330
Commercial \| Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	1	1	3	4
Average Recorded Investment	391	340	402	277
Commercial \| Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	0	0	0	0
Average Recorded Investment	33	40	39	53
Retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	6	8	22	23
Average Recorded Investment	748	800	719	776
Retail \| Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	0	1	3	3
Average Recorded Investment	137	169	128	161
Retail \| Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	1	2	4	5
Average Recorded Investment	125	153	124	151
Retail \| Home equity lines of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	2	2	5	5
Average Recorded Investment	192	192	178	182
Retail \| Home equity loans serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	0	1	2	3
Average Recorded Investment	51	64	51	64
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	21	17	18	14
Retail \| Education
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	3	1	7	5
Average Recorded Investment	178	159	178	157
Retail \| Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	0	0	1	1
Average Recorded Investment	25	25	23	25
Retail \| Other retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	0	1	0	1
Average Recorded Investment	$ 10	$ 12	$ 10	$ 13

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- Definition

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://www.xbrl.org/2003/role/presentationRef
-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://www.xbrl.org/2003/role/presentationRef
-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.

+ References

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