ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Additional Impaired Loan Information (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	$ 10	$ 9	$ 20	$ 18
Average Recorded Investment	1,104	1,259	1,069	1,231
Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	2	1	4	2
Average Recorded Investment	368	469	343	455
Commercial \| Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	2	1	4	2
Average Recorded Investment	332	431	311	414
Commercial \| Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	0	0	0	0
Average Recorded Investment	36	38	32	41
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	8	16	16
Average Recorded Investment	736	790	726	776
Retail \| Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	2	2	3	3
Average Recorded Investment	152	182	149	178
Retail \| Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	1	1	3	3
Average Recorded Investment	112	141	112	140
Retail \| Home equity lines of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	2	1	4	3
Average Recorded Investment	198	203	192	197
Retail \| Home equity loans serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	0	1	1	2
Average Recorded Investment	46	54	47	54
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	22	20	21	18
Retail \| Education
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	2	2	4	4
Average Recorded Investment	165	146	165	146
Retail \| Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	1	1	1	1
Average Recorded Investment	24	25	23	24
Retail \| Other retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	0	0	0	0
Average Recorded Investment	$ 8	$ 10	$ 8	$ 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://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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