MORTGAGE BANKING - Changes Related to MSRs - Fair Value Method (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
MSRs
Fair value as of beginning of period			$ 642
Fair value at end of the period	$ 568		568
Residential mortgages
MSRs
Fair value as of beginning of period	577	$ 563	642	$ 600
Amounts capitalized	86	57	153	92
Changes in unpaid principal balance during the period	(46)	(31)	(86)	(57)
Changes in fair value during the period	(49)	(58)	(331)	(104)
Fair value at end of the period	568	531	568	531
Cumulative Effect, Period Of Adoption, Adjustment \| Residential mortgages
MSRs
Fair value as of beginning of period	0	0	190	0
Cumulative Effect, Period Of Adoption, Adjusted Balance \| Residential mortgages
MSRs
Fair value as of beginning of period	$ 577	$ 563	$ 832	$ 600

Servicing Asset At Fair Value, Amount Capitalized

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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 45
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-URI http://asc.fasb.org/extlink&oid=6469459&loc=d3e122501-111745

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

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Amount of increase (decrease) from changes in fair value classified as other for a contract to service financial assets under which the benefits of servicing are expected to more than adequately compensate the servicer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 50
-Paragraph 3
-Subparagraph (a)(4)(ii)
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Amount of increase (decrease) in fair value from changes classified as other for a contract to service financial assets under which the benefits of servicing are expected to more than adequately compensate the servicer.

+ References

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-Topic 860
-SubTopic 50
-Section 50
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