BANK LOANS, NET, Allowance for Loan Losses (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Mar. 31, 2024	Sep. 30, 2023
Net (charge-offs)/recoveries:
Increase (decrease) in allowance for credit losses	$ (15)		$ (18)
Loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	471	$ 415	474	$ 396
Provision/(benefit) for credit losses	(10)		23
Net (charge-offs)/recoveries:
Charge-offs	(7)	(15)	(45)	(40)
Recoveries	1	0	3	3
Net (charge-offs)/recoveries	(6)	(15)	(42)	(37)
Foreign exchange translation adjustment	1	2	1	1
Balance at end of period	$ 456	$ 456	$ 456	$ 456
ACL by loan portfolio segment as a % of total ACL	100.00%	100.00%	100.00%	100.00%
Unfunded lending commitments
Net (charge-offs)/recoveries:
Unfunded lending commitments	$ 22		$ 22		$ 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		$ 54		$ 96
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	6	5	7	3
Provision/(benefit) for credit losses	(1)		(2)
Net (charge-offs)/recoveries:
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Net (charge-offs)/recoveries	0	0	0	0
Foreign exchange translation adjustment	0	0	0	0
Balance at end of period	$ 5	$ 5	$ 5	$ 5
ACL by loan portfolio segment as a % of total ACL	1.10%	1.10%	1.10%	1.10%
SBL | Provision/(benefit) for credit losses | Loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Provision/(benefit) for credit losses		$ 0		$ 2
C&I loans
Net (charge-offs)/recoveries:
Charge-offs			$ (37)
C&I loans | Loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	$ 196	219	214	226
Provision/(benefit) for credit losses	(20)		(9)
Net (charge-offs)/recoveries:
Charge-offs	(6)	(6)	(37)	(30)
Recoveries	0	0	2	0
Net (charge-offs)/recoveries	(6)	(6)	(35)	(30)
Foreign exchange translation adjustment	0	1	0	0
Balance at end of period	$ 170	$ 206	$ 170	$ 206
ACL by loan portfolio segment as a % of total ACL	37.30%	45.30%	37.30%	45.30%
C&I loans | Provision/(benefit) for credit losses | Loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Provision/(benefit) for credit losses		$ (8)		$ 10
CRE loans
Net (charge-offs)/recoveries:
Charge-offs			$ (8)
CRE loans | Loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	$ 181	100	161	87
Provision/(benefit) for credit losses	16		43
Net (charge-offs)/recoveries:
Charge-offs	(1)	(9)	(8)	(10)
Recoveries	0	0	0	3
Net (charge-offs)/recoveries	(1)	(9)	(8)	(7)
Foreign exchange translation adjustment	1	1	1	1
Balance at end of period	$ 197	$ 147	$ 197	$ 147
ACL by loan portfolio segment as a % of total ACL	43.20%	32.20%	43.20%	32.20%
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		$ 55		$ 66
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	$ 19	15	16	21
Provision/(benefit) for credit losses	1		4
Net (charge-offs)/recoveries:
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Net (charge-offs)/recoveries	0	0	0	0
Foreign exchange translation adjustment	0	0	0	0
Balance at end of period	$ 20	$ 16	$ 20	$ 16
ACL by loan portfolio segment as a % of total ACL	4.40%	3.50%	4.40%	3.50%
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		$ 1		$ (5)
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	$ 67	74	74	57
Provision/(benefit) for credit losses	(6)		(13)
Net (charge-offs)/recoveries:
Charge-offs	0	0	0	0
Recoveries	1	0	1	0
Net (charge-offs)/recoveries	1	0	1	0
Foreign exchange translation adjustment	0	0	0	0
Balance at end of period	$ 62	$ 80	$ 62	$ 80
ACL by loan portfolio segment as a % of total ACL	13.60%	17.50%	13.60%	17.50%
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		$ 6		$ 23
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	2	2
Provision/(benefit) for credit losses	0		0
Net (charge-offs)/recoveries:
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Net (charge-offs)/recoveries	0	0	0	0
Foreign exchange translation adjustment	0	0	0	0
Balance at end of period	$ 2	$ 2	$ 2	$ 2
ACL by loan portfolio segment as a % of total ACL	0.40%	0.40%	0.40%	0.40%
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		$ 0