BANK LOANS, NET, Allowance for Loan Losses (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Net (charge-offs)/recoveries:
Increase (decrease) in allowance for credit losses	$ 78
Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year	396	$ 320	$ 354
Initial allowance on acquired PCD loans		3
Provision/(benefit) for credit losses	132	100	(32)
Net (charge-offs)/recoveries:
Charge-offs	(58)	(32)	(14)
Recoveries	4	6	1
Net (charge-offs)/recoveries	(54)	(26)	(13)
Foreign exchange translation adjustment	0	(1)	2
Balance at end of period	$ 474	$ 396	$ 320
ACL by loan portfolio segment as a % of total ACL	100.00%	100.00%	100.00%
Total loans held for investment | Cumulative adjustments for changes in accounting principles
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year			$ 9
Unfunded lending commitments
Net (charge-offs)/recoveries:
Allowance, unfunded lending commitment	$ 22	$ 19	13
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		74
SBL | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year	3	4	5
Initial allowance on acquired PCD loans		0
Provision/(benefit) for credit losses	4	(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	$ 7	$ 3	$ 4
ACL by loan portfolio segment as a % of total ACL	1.50%	0.80%	1.30%
SBL | Total loans held for investment | Cumulative adjustments for changes in accounting principles
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year			$ (2)
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		(3)
C&I | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year	$ 226	191	200
Initial allowance on acquired PCD loans		1
Provision/(benefit) for credit losses	32	62	(25)
Net (charge-offs)/recoveries:
Charge-offs	(45)	(28)	(4)
Recoveries	1	0	0
Net (charge-offs)/recoveries	(44)	(28)	(4)
Foreign exchange translation adjustment	0	0	1
Balance at end of period	$ 214	$ 226	$ 191
ACL by loan portfolio segment as a % of total ACL	45.10%	57.00%	59.70%
C&I | Total loans held for investment | Cumulative adjustments for changes in accounting principles
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year			$ 19
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		57
CRE loans | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year	$ 87	66	81
Initial allowance on acquired PCD loans		2
Provision/(benefit) for credit losses	84	19	5
Net (charge-offs)/recoveries:
Charge-offs	(13)	(4)	(10)
Recoveries	3	5	0
Net (charge-offs)/recoveries	(10)	1	(10)
Foreign exchange translation adjustment	0	(1)	1
Balance at end of period	$ 161	$ 87	$ 66
ACL by loan portfolio segment as a % of total ACL	34.00%	22.00%	20.60%
CRE loans | Total loans held for investment | Cumulative adjustments for changes in accounting principles
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year			$ (11)
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		0
REIT loans | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year	$ 21	22	36
Initial allowance on acquired PCD loans		0
Provision/(benefit) for credit losses	(5)	(1)	(5)
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	$ 16	$ 21	$ 22
ACL by loan portfolio segment as a % of total ACL	3.40%	5.30%	6.90%
REIT loans | Total loans held for investment | Cumulative adjustments for changes in accounting principles
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year			$ (9)
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		(1)
Residential mortgage loans | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year	$ 57	35	18
Initial allowance on acquired PCD loans		0
Provision/(benefit) for credit losses	17	21	(8)
Net (charge-offs)/recoveries:
Charge-offs	0	0	0
Recoveries	0	1	1
Net (charge-offs)/recoveries	0	1	1
Foreign exchange translation adjustment	0	0	0
Balance at end of period	$ 74	$ 57	$ 35
ACL by loan portfolio segment as a % of total ACL	15.60%	14.40%	10.90%
Residential mortgage loans | Total loans held for investment | Cumulative adjustments for changes in accounting principles
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year			$ 24
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		21
Tax-exempt loans | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year	$ 2	2	14
Initial allowance on acquired PCD loans		0
Provision/(benefit) for credit losses	0	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%	0.50%	0.60%
Tax-exempt loans | Total loans held for investment | Cumulative adjustments for changes in accounting principles
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year			$ (12)
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