INVESTMENT SECURITIES - Gains (Losses) Within Investment Income (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Investments, Debt and Equity Securities [Abstract]
Gross gains on investment securities	$ 71	$ 2	$ 180	$ 71
Gross losses on investment securities	(1)	0	(1)	(18)
Net (losses) gains on equity securities	(174)	643	(197)	643
Net (losses) gains on investment securities	$ (104)	$ 645	$ (18)	$ 696

Amount of realized gain (loss) from sale of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 321
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=123583765&loc=SL75117546-209714

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(3)(d))
-URI http://asc.fasb.org/extlink&oid=120400993&loc=SL114874131-224263

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