BANK LOANS, NET, Allowance for Loan Losses (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Sep. 30, 2023
Net (charge-offs)/recoveries:
Increase (decrease) in allowance for credit losses	$ 5
Loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	474	$ 396
Provision/(benefit) for credit losses	12
Net (charge-offs)/recoveries:
Charge-offs	(8)	(5)
Recoveries	0	3
Net (charge-offs)/recoveries	(8)	(2)
Foreign exchange translation adjustment	1	0
Balance at end of period	$ 479	$ 408
ACL by loan portfolio segment as a % of total ACL	100.00%	100.00%
Unfunded lending commitments
Net (charge-offs)/recoveries:
Unfunded lending commitments	$ 20		$ 22
Provision/(benefit) for credit losses | Loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Provision/(benefit) for credit losses		$ 14
SBL
Net (charge-offs)/recoveries:
Charge-offs	0
SBL | Loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	7	3
Provision/(benefit) for credit losses	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	$ 7	$ 4
ACL by loan portfolio segment as a % of total ACL	1.50%	1.00%
SBL | Provision/(benefit) for credit losses | Loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Provision/(benefit) for credit losses		$ 1
C&I
Net (charge-offs)/recoveries:
Charge-offs	$ (6)
C&I | Loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	214	226
Provision/(benefit) for credit losses	3
Net (charge-offs)/recoveries:
Charge-offs	(6)	(4)
Recoveries	0	0
Net (charge-offs)/recoveries	(6)	(4)
Foreign exchange translation adjustment	0	0
Balance at end of period	$ 211	$ 222
ACL by loan portfolio segment as a % of total ACL	44.10%	54.40%
C&I | Provision/(benefit) for credit losses | Loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Provision/(benefit) for credit losses		$ 0
CRE loans
Net (charge-offs)/recoveries:
Charge-offs	$ (2)
CRE loans | Loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	161	87
Provision/(benefit) for credit losses	14
Net (charge-offs)/recoveries:
Charge-offs	(2)	(1)
Recoveries	0	3
Net (charge-offs)/recoveries	(2)	2
Foreign exchange translation adjustment	1	0
Balance at end of period	$ 174	$ 91
ACL by loan portfolio segment as a % of total ACL	36.30%	22.30%
CRE loans | Provision/(benefit) for credit losses | Loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Provision/(benefit) for credit losses		$ 2
REIT loans
Net (charge-offs)/recoveries:
Charge-offs	$ 0
REIT loans | Loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	16	21
Provision/(benefit) for credit losses	1
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	$ 17	$ 15
ACL by loan portfolio segment as a % of total ACL	3.50%	3.70%
REIT loans | Provision/(benefit) for credit losses | Loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Provision/(benefit) for credit losses		$ (6)
Residential mortgage loans
Net (charge-offs)/recoveries:
Charge-offs	$ 0
Residential mortgage loans | Loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	74	57
Provision/(benefit) for credit losses	(6)
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	$ 68	$ 74
ACL by loan portfolio segment as a % of total ACL	14.20%	18.10%
Residential mortgage loans | Provision/(benefit) for credit losses | Loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Provision/(benefit) for credit losses		$ 17
Tax-exempt loans
Net (charge-offs)/recoveries:
Charge-offs	$ 0
Tax-exempt loans | Loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	2	2
Provision/(benefit) for credit losses	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
ACL by loan portfolio segment as a % of total ACL	0.40%	0.50%
Tax-exempt loans | Provision/(benefit) for credit losses | Loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Provision/(benefit) for credit losses		$ 0