FAIR VALUE, Financial Instruments Measured at Fair Value on a Nonrecurring Basis (Details) - Nonrecurring - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Mar. 31, 2020	Sep. 30, 2019
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis [Line Items]
Other assets: other real estate owned	$ 2	$ 1
Level 2
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis [Line Items]
Other assets: other real estate owned	2	1
Level 3
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis [Line Items]
Other assets: other real estate owned	0	0
Loans held for sale
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis [Line Items]
Bank loans, net	67	66
Loans held for sale | Level 2
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis [Line Items]
Bank loans, net	67	66
Loans held for sale | Level 3
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis [Line Items]
Bank loans, net	0	0
Discounted cash flow | Bank loans: impaired loans - residential
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis [Line Items]
Bank loans, net	18	21
Discounted cash flow | Bank loans: impaired loans - residential | Level 2
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis [Line Items]
Bank loans, net	4	7
Discounted cash flow | Bank loans: impaired loans - residential | Level 3
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis [Line Items]
Bank loans, net	14	14
Collateral or discounted cash flow | Bank loans: impaired loans - corporate
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis [Line Items]
Bank loans, net	10	21
Collateral or discounted cash flow | Bank loans: impaired 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: impaired loans - corporate | Level 3
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis [Line Items]
Bank loans, net	$ 10	$ 21
Minimum | Discounted cash flow | Bank loans: impaired loans - residential | Level 3
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis [Line Items]
Prepayment term	7 years	7 years
Maximum | Discounted cash flow | Bank loans: impaired loans - residential | Level 3
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis [Line Items]
Prepayment term	12 years	12 years
Weighted average | Discounted cash flow | Bank loans: impaired loans - residential | Level 3
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis [Line Items]
Prepayment term	10 years 4 months 24 days	10 years 4 months 24 days