FAIR VALUE, Financial Instruments Measured at Fair Value on a Nonrecurring Basis (Details) - Nonrecurring $ in Millions	6 Months Ended	12 Months Ended
	Mar. 31, 2024 USD ($)	Sep. 30, 2023 USD ($)
Loans held for sale
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis [Line Items]
Bank loans, net	$ 41	$ 2
Loans held for sale | Level 2
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis [Line Items]
Bank loans, net	41	2
Loans held for sale | Level 3
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis [Line Items]
Bank loans, net	0	0
Collateral or discounted cash flow | Bank loans - residential
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis [Line Items]
Bank loans, net	10	10
Collateral or discounted cash flow | Bank loans - residential | Level 2
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis [Line Items]
Bank loans, net	2	2
Collateral or discounted cash flow | Bank loans - residential | Level 3
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis [Line Items]
Bank loans, net	8	8
Collateral or discounted cash flow | Bank loans - corporate
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis [Line Items]
Bank loans, net	125	84
Collateral or discounted cash flow | Bank loans - corporate | Level 2
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis [Line Items]
Bank loans, net	0	0
Collateral or discounted cash flow | Bank loans - corporate | Level 3
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis [Line Items]
Bank loans, net	$ 125	$ 84
Minimum | Collateral or discounted cash flow | Bank loans - residential | Prepayment rate
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis [Line Items]
Prepayment term (in years)	7 years	7 years
Minimum | Collateral or discounted cash flow | Bank loans - corporate | Recovery rate
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis [Line Items]
Bank loans, net, measurement input (as a percent)	0.33	0.22
Maximum | Collateral or discounted cash flow | Bank loans - residential | Prepayment rate
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis [Line Items]
Prepayment term (in years)	12 years	12 years
Maximum | Collateral or discounted cash flow | Bank loans - corporate | Recovery rate
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis [Line Items]
Bank loans, net, measurement input (as a percent)	0.59	0.65
Weighted average | Collateral or discounted cash flow | Bank loans - residential | Prepayment rate
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis [Line Items]
Prepayment term (in years)	10 years 3 months 18 days	10 years 3 months 18 days
Weighted average | Collateral or discounted cash flow | Bank loans - corporate | Recovery rate
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis [Line Items]
Bank loans, net, measurement input (as a percent)	0.56	0.53