AVAILABLE-FOR-SALE SECURITIES, Narrative (Details)	3 Months Ended		9 Months Ended
	Jun. 30, 2024 USD ($) investmentPosition	Jun. 30, 2023 USD ($)	Jun. 30, 2024 USD ($) investmentPosition	Jun. 30, 2023 USD ($)	Sep. 30, 2023 USD ($)
Debt Securities, Available-for-sale [Line Items]
Accrued interest on available-for-sale securities, location	Other receivables, net		Other receivables, net		Other receivables, net
Duration, available for sale securities (in years)			4 years
Fair value	$ 8,530,000,000		$ 8,530,000,000		$ 9,181,000,000
Bank
Debt Securities, Available-for-sale [Line Items]
Accrued interest excluded	$ 28,000,000		$ 28,000,000		28,000,000
Number of available-for-sale investment positions determined to be in an unrealized loss position | investmentPosition	1,000		1,000
Number of available-for-sale investment positions determined to be in an unrealized loss position continuously for less than 12 months | investmentPosition	17		17
Number of available-for-sale investment positions determined to be in an unrealized loss position continuously for 12 months or more | investmentPosition	983		983
Amortized cost basis	$ 9,444,000,000		$ 9,444,000,000		10,433,000,000
Fair value	8,530,000,000		8,530,000,000		$ 9,181,000,000
Proceeds from sales of available-for-sale securities	0	$ 0	0	$ 0
Bank | Federal National Mortgage Association (FNMA)
Debt Securities, Available-for-sale [Line Items]
Amortized cost basis	4,360,000,000		4,360,000,000
Fair value	3,860,000,000		3,860,000,000
Bank | Federal Home Loan Mortgage Corporation (FHLMC)
Debt Securities, Available-for-sale [Line Items]
Amortized cost basis	2,660,000,000		2,660,000,000
Fair value	$ 2,330,000,000		$ 2,330,000,000