INVESTMENT SECURITIES AND OTHER SECURITIES - Narrative (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Mar. 31, 2020	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Transfer of securities from available-for-sale to held-to-maturity	$ 1,500	$ 1,520	$ 0
Unrealized Net Losses Recognized In OCI At Time Of Transfer Of Available For Sale Securities Transferred To Held To Maturity Securities	$ 22
Pledged Financial Instruments, Not Separately Reported, Securities for Other Debt Facilities		$ 16,800		$ 3,800

The carrying amount as of the date of the latest financial statement presented of securities which are owned but transferred to serve as collateral for debt facilities other than FHLB loans, repurchase agreements and letter of credit arrangements, and for which the transferee does not have the right by contract or custom to sell or re-pledge them to an unrelated party.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (b)(1)(i)
-URI http://asc.fasb.org/extlink&oid=109249958&loc=SL6224234-111729

Name:	us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesForOtherDebtFacilities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant