FAIR VALUE MEASUREMENTS - Changes in Fair Value for Assets with Fair Value Option (Details) - Level 2 - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Residential loans held for sale \| Mortgage banking fees
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value	$ 46	$ (4)	$ 149	$ 5
Commercial and commercial real estate loans held for sale, at fair value \| Other income
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value	$ 6	$ 0	$ (2)	$ 4

For each line item in the statement of financial position, the amounts of gains and losses from fair value changes included in earnings.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 30
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=75031198&loc=d3e14172-108612

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration