BANK LOANS, NET, Changes in the Allowance for Loan Losses (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Sep. 30, 2021
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	$ 320
Net (charge-offs)/recoveries:
Balance at end of period	308
Unfunded lending commitments	12		$ 13
Decrease in allowance for credit losses	12
Loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	320	$ 354
Provision/(benefit) for credit losses	(11)	14
Net (charge-offs)/recoveries:
Charge-offs	(2)	0
Recoveries	1	0
Net (charge-offs)/recoveries	(1)	0
Foreign exchange translation adjustment	0	1
Balance at end of period	308	378
Loans held for investment | Impact of CECL adoption
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period		9
Loans held for investment | C&I loans
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	191	200
Provision/(benefit) for credit losses	(10)	(22)
Net (charge-offs)/recoveries:
Charge-offs	(2)	0
Recoveries	0	0
Net (charge-offs)/recoveries	(2)	0
Foreign exchange translation adjustment	0	1
Balance at end of period	179	198
Loans held for investment | C&I loans | Impact of CECL adoption
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period		19
Loans held for investment | CRE loans
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	66	81
Provision/(benefit) for credit losses	6	42
Net (charge-offs)/recoveries:
Charge-offs	0	0
Recoveries	0	0
Net (charge-offs)/recoveries	0	0
Foreign exchange translation adjustment	0	0
Balance at end of period	72	112
Loans held for investment | CRE loans | Impact of CECL adoption
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period		(11)
Loans held for investment | REIT loans
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	22	36
Provision/(benefit) for credit losses	0	3
Net (charge-offs)/recoveries:
Charge-offs	0	0
Recoveries	0	0
Net (charge-offs)/recoveries	0	0
Foreign exchange translation adjustment	0	0
Balance at end of period	22	30
Loans held for investment | REIT loans | Impact of CECL adoption
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period		(9)
Loans held for investment | Tax-exempt loans
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	2	14
Provision/(benefit) for credit losses	0	0
Net (charge-offs)/recoveries:
Charge-offs	0	0
Recoveries	0	0
Net (charge-offs)/recoveries	0	0
Foreign exchange translation adjustment	0	0
Balance at end of period	2	2
Loans held for investment | Tax-exempt loans | Impact of CECL adoption
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period		(12)
Loans held for investment | Residential mortgage loans
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	35	18
Provision/(benefit) for credit losses	(6)	(9)
Net (charge-offs)/recoveries:
Charge-offs	0	0
Recoveries	1	0
Net (charge-offs)/recoveries	1	0
Foreign exchange translation adjustment	0	0
Balance at end of period	30	33
Loans held for investment | Residential mortgage loans | Impact of CECL adoption
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period		24
Loans held for investment | SBL and other
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	4	5
Provision/(benefit) for credit losses	(1)	0
Net (charge-offs)/recoveries:
Charge-offs	0	0
Recoveries	0	0
Net (charge-offs)/recoveries	0	0
Foreign exchange translation adjustment	0	0
Balance at end of period	$ 3	3
Loans held for investment | SBL and other | Impact of CECL adoption
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period		$ (2)