MORTGAGE BANKING - Economic Assumptions Used to Estimate Value of MSRs (Details) - Residential mortgages - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Servicing Assets and Servicing Liabilities at Fair Value, Assumptions Used to Estimate Fair Value [Abstract]
Fair value	$ 218	$ 182
Weighted-average life (in years)	5 years 10 months 24 days	5 years 8 months 12 days
Weighted-average constant prepayment rate	10.00%	10.80%
Weighted-average discount rate	9.90%	9.70%
Minimum
Servicing Assets and Servicing Liabilities at Fair Value, Assumptions Used to Estimate Fair Value [Abstract]
Weighted-average life (in years)	2 years 3 months 18 days	2 years 7 months 6 days
Weighted-average constant prepayment rate	6.60%	8.80%
Weighted-average discount rate	9.10%	9.10%
Maximum
Servicing Assets and Servicing Liabilities at Fair Value, Assumptions Used to Estimate Fair Value [Abstract]
Weighted-average life (in years)	8 years 4 months 24 days	7 years 3 months 18 days
Weighted-average constant prepayment rate	20.10%	22.30%
Weighted-average discount rate	12.10%	12.10%

Fair value of an asset representing net future revenue from contractually specified servicing fees, late charges, and other ancillary revenues, in excess of future costs related to servicing arrangements.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=68078379&loc=d3e122625-111746

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- References

No definition available.

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- Definition

Discount rate used to estimate the fair value of servicing assets and servicing liabilities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 50
-Paragraph 2
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=68078379&loc=d3e122596-111746

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- Definition

Estimated rate of prepayments of principal on servicing assets and servicing liabilities.

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- Definition

Weighted average life of financial assets and financial liabilities, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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