COLLATERALIZED AGREEMENTS AND FINANCINGS, Collateral (Details) - USD ($) $ in Millions	Mar. 31, 2021	Sep. 30, 2020
Collateral Received that Can be Resold or Repledged [Abstract]
Collateral we received that was available to be delivered or repledged	$ 3,225	$ 2,869
Collateral that we delivered or repledged	$ 886	$ 788

Fair value of the collateral, or portion thereof, that has been re-pledged, delivered, or otherwise used.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=109249958&loc=SL6224234-111729

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