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, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	$ 11	$ 10	$ 22	$ 20
Average Recorded Investment	1,019	1,104	1,003	1,069
Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	3	2	6	4
Average Recorded Investment	337	368	331	343
Commercial \| Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	2	2	5	4
Average Recorded Investment	310	332	304	311
Commercial \| Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	1	0	1	0
Average Recorded Investment	27	36	27	32
Retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	8	8	16	16
Average Recorded Investment	682	736	672	726
Retail \| Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	2	2	3	3
Average Recorded Investment	145	152	142	149
Retail \| Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	1	1	3	3
Average Recorded Investment	97	112	97	112
Retail \| Home equity lines of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	2	2	4	4
Average Recorded Investment	199	198	193	192
Retail \| Home equity loans serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	0	0	1	1
Average Recorded Investment	36	46	37	47
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	7	9	7	9
Retail \| Automobile
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	0	0	0	0
Average Recorded Investment	22	22	21	21
Retail \| Education
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	2	2	4	4
Average Recorded Investment	145	165	145	165
Retail \| Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	1	1	1	1
Average Recorded Investment	25	24	24	23
Retail \| Other retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	0	0	0	0
Average Recorded Investment	$ 6	$ 8	$ 6	$ 8

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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://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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