BANK LOANS, NET, Allowance for Loan Losses (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Net (charge-offs)/recoveries:
Decrease in allowance for credit losses	$ 17
Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year	474	$ 396	$ 320
Initial allowance on acquired PCD loans			3
Provision/(benefit) for credit losses	45		100
Net (charge-offs)/recoveries:
Charge-offs	(66)	(58)	(32)
Recoveries	4	4	6
Net (charge-offs)/recoveries	(62)	(54)	(26)
Foreign exchange translation adjustment	0	0	(1)
Balance at end of period	$ 457	$ 474	$ 396
ACL by loan portfolio segment as a % of total ACL	100.00%	100.00%	100.00%
Unfunded lending commitments
Net (charge-offs)/recoveries:
Allowance, unfunded lending commitment	$ 22	$ 22	$ 19
Financial Asset Acquired and No Credit Deterioration | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Provision/(benefit) for credit losses			26
Financial Asset Originated | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Provision/(benefit) for credit losses		132	74
SBL
Net (charge-offs)/recoveries:
Charge-offs	0
SBL | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year	7	3	4
Initial allowance on acquired PCD loans			0
Provision/(benefit) for credit losses	(1)		(1)
Net (charge-offs)/recoveries:
Charge-offs	0	0	0
Recoveries	0	0	0
Net (charge-offs)/recoveries	0	0	0
Foreign exchange translation adjustment	0	0	0
Balance at end of period	$ 6	$ 7	$ 3
ACL by loan portfolio segment as a % of total ACL	1.30%	1.50%	0.80%
SBL | Financial Asset Acquired and No Credit Deterioration | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Provision/(benefit) for credit losses			$ 2
SBL | Financial Asset Originated | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Provision/(benefit) for credit losses		$ 4	(3)
C&I
Net (charge-offs)/recoveries:
Charge-offs	$ (45)
C&I | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year	214	226	191
Initial allowance on acquired PCD loans			1
Provision/(benefit) for credit losses	1		62
Net (charge-offs)/recoveries:
Charge-offs	(45)	(45)	(28)
Recoveries	3	1	0
Net (charge-offs)/recoveries	(42)	(44)	(28)
Foreign exchange translation adjustment	0	0	0
Balance at end of period	$ 173	$ 214	$ 226
ACL by loan portfolio segment as a % of total ACL	38.00%	45.10%	57.00%
C&I | Financial Asset Acquired and No Credit Deterioration | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Provision/(benefit) for credit losses			$ 5
C&I | Financial Asset Originated | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Provision/(benefit) for credit losses		$ 32	57
CRE loans
Net (charge-offs)/recoveries:
Charge-offs	$ (21)
CRE loans | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year	161	87	66
Initial allowance on acquired PCD loans			2
Provision/(benefit) for credit losses	48		19
Net (charge-offs)/recoveries:
Charge-offs	(21)	(13)	(4)
Recoveries	0	3	5
Net (charge-offs)/recoveries	(21)	(10)	1
Foreign exchange translation adjustment	0	0	(1)
Balance at end of period	$ 188	$ 161	$ 87
ACL by loan portfolio segment as a % of total ACL	41.10%	34.00%	22.00%
CRE loans | Financial Asset Acquired and No Credit Deterioration | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Provision/(benefit) for credit losses			$ 19
CRE loans | Financial Asset Originated | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Provision/(benefit) for credit losses		$ 84	0
REIT loans
Net (charge-offs)/recoveries:
Charge-offs	$ 0
REIT loans | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year	16	21	22
Initial allowance on acquired PCD loans			0
Provision/(benefit) for credit losses	7		(1)
Net (charge-offs)/recoveries:
Charge-offs	0	0	0
Recoveries	0	0	0
Net (charge-offs)/recoveries	0	0	0
Foreign exchange translation adjustment	0	0	0
Balance at end of period	$ 23	$ 16	$ 21
ACL by loan portfolio segment as a % of total ACL	5.00%	3.40%	5.30%
REIT loans | Financial Asset Acquired and No Credit Deterioration | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Provision/(benefit) for credit losses			$ 0
REIT loans | Financial Asset Originated | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Provision/(benefit) for credit losses		$ (5)	(1)
Residential mortgage loans
Net (charge-offs)/recoveries:
Charge-offs	$ 0
Residential mortgage loans | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year	74	57	35
Initial allowance on acquired PCD loans			0
Provision/(benefit) for credit losses	(10)		21
Net (charge-offs)/recoveries:
Charge-offs	0	0	0
Recoveries	1	0	1
Net (charge-offs)/recoveries	1	0	1
Foreign exchange translation adjustment	0	0	0
Balance at end of period	$ 65	$ 74	$ 57
ACL by loan portfolio segment as a % of total ACL	14.20%	15.60%	14.40%
Residential mortgage loans | Financial Asset Acquired and No Credit Deterioration | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Provision/(benefit) for credit losses			$ 0
Residential mortgage loans | Financial Asset Originated | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Provision/(benefit) for credit losses		$ 17	21
Tax-exempt loans
Net (charge-offs)/recoveries:
Charge-offs	$ 0
Tax-exempt loans | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year	2	2	2
Initial allowance on acquired PCD loans			0
Provision/(benefit) for credit losses	0		0
Net (charge-offs)/recoveries:
Charge-offs	0	0	0
Recoveries	0	0	0
Net (charge-offs)/recoveries	0	0	0
Foreign exchange translation adjustment	0	0	0
Balance at end of period	$ 2	$ 2	$ 2
ACL by loan portfolio segment as a % of total ACL	0.40%	0.40%	0.50%
Tax-exempt loans | Financial Asset Acquired and No Credit Deterioration | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Provision/(benefit) for credit losses			$ 0
Tax-exempt loans | Financial Asset Originated | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Provision/(benefit) for credit losses		$ 0	$ 0