FAIR VALUE, Financial Instruments Measured at Fair Value on a Nonrecurring Basis (Details) - Nonrecurring - USD ($) $ in Millions	9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Sep. 30, 2019
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis [Line Items]
Other assets: other real estate owned	$ 1		$ 1
Level 2
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis [Line Items]
Other assets: other real estate owned	1		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	21		66
Loans held for sale | Level 2
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis [Line Items]
Bank loans, net	21		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	5		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	13		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	7		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	$ 7		$ 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