BANK LOANS, NET, Allowance for Loan Losses (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year	$ 320
Net (charge-offs)/recoveries:
Balance at end of period	396	$ 320
Increase (decrease) in allowance for credit losses	76
Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year	320	354	$ 218
Provision/(benefit) for credit losses	100	(32)	233
Net (charge-offs)/recoveries:
Charge-offs	(32)	(14)	(100)
Recoveries	6	1	2
Net (charge-offs)/recoveries	(26)	(13)	(98)
Foreign exchange translation adjustment	(1)	2	1
Balance at end of period	$ 396	$ 320	$ 354
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	$ 19	$ 13	$ 12
Financial Asset Acquired with Credit Deterioration | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year	3
Net (charge-offs)/recoveries:
Balance at end of period		3
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
Cumulative adjustments for changes in accounting principles | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year		9
Net (charge-offs)/recoveries:
Balance at end of period			9
SBL | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year	4	5	5
Provision/(benefit) for credit losses	(1)	1	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	$ 3	$ 4	$ 5
ACL by loan portfolio segment as a % of total ACL	0.80%	1.30%	1.40%
SBL | Financial Asset Acquired with Credit Deterioration | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year	$ 0
Net (charge-offs)/recoveries:
Balance at end of period		$ 0
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)
SBL | Cumulative adjustments for changes in accounting principles | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year		(2)
Net (charge-offs)/recoveries:
Balance at end of period			$ (2)
C&I | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year	191	200	139
Provision/(benefit) for credit losses	62	(25)	157
Net (charge-offs)/recoveries:
Charge-offs	(28)	(4)	(96)
Recoveries	0	0	0
Net (charge-offs)/recoveries	(28)	(4)	(96)
Foreign exchange translation adjustment	0	1	0
Balance at end of period	$ 226	$ 191	$ 200
ACL by loan portfolio segment as a % of total ACL	57.00%	59.70%	56.40%
C&I | Financial Asset Acquired with Credit Deterioration | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year	$ 1
Net (charge-offs)/recoveries:
Balance at end of period		$ 1
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
C&I | Cumulative adjustments for changes in accounting principles | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year		19
Net (charge-offs)/recoveries:
Balance at end of period			$ 19
CRE loans | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year	66	81	34
Provision/(benefit) for credit losses	19	5	48
Net (charge-offs)/recoveries:
Charge-offs	(4)	(10)	(2)
Recoveries	5	0	0
Net (charge-offs)/recoveries	1	(10)	(2)
Foreign exchange translation adjustment	(1)	1	1
Balance at end of period	$ 87	$ 66	$ 81
ACL by loan portfolio segment as a % of total ACL	22.00%	20.60%	22.90%
CRE loans | Financial Asset Acquired with Credit Deterioration | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year	$ 2
Net (charge-offs)/recoveries:
Balance at end of period		$ 2
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
CRE loans | Cumulative adjustments for changes in accounting principles | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year		(11)
Net (charge-offs)/recoveries:
Balance at end of period			$ (11)
REIT loans | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year	22	36	15
Provision/(benefit) for credit losses	(1)	(5)	23
Net (charge-offs)/recoveries:
Charge-offs	0	0	(2)
Recoveries	0	0	0
Net (charge-offs)/recoveries	0	0	(2)
Foreign exchange translation adjustment	0	0	0
Balance at end of period	$ 21	$ 22	$ 36
ACL by loan portfolio segment as a % of total ACL	5.30%	6.90%	10.20%
REIT loans | Financial Asset Acquired with Credit Deterioration | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year	$ 0
Net (charge-offs)/recoveries:
Balance at end of period		$ 0
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)
REIT loans | Cumulative adjustments for changes in accounting principles | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year		(9)
Net (charge-offs)/recoveries:
Balance at end of period			$ (9)
Residential mortgage loans | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year	35	18	16
Provision/(benefit) for credit losses	21	(8)	0
Net (charge-offs)/recoveries:
Charge-offs	0	0	0
Recoveries	1	1	2
Net (charge-offs)/recoveries	1	1	2
Foreign exchange translation adjustment	0	0	0
Balance at end of period	$ 57	$ 35	$ 18
ACL by loan portfolio segment as a % of total ACL	14.40%	10.90%	5.10%
Residential mortgage loans | Financial Asset Acquired with Credit Deterioration | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year	$ 0
Net (charge-offs)/recoveries:
Balance at end of period		$ 0
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
Residential mortgage loans | Cumulative adjustments for changes in accounting principles | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year		24
Net (charge-offs)/recoveries:
Balance at end of period			$ 24
Tax-exempt loans | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year	2	14	9
Provision/(benefit) for credit losses	0	0	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	$ 2	$ 2	$ 14
ACL by loan portfolio segment as a % of total ACL	0.50%	0.60%	4.00%
Tax-exempt loans | Financial Asset Acquired with Credit Deterioration | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year	$ 0
Net (charge-offs)/recoveries:
Balance at end of period		$ 0
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
Tax-exempt loans | Cumulative adjustments for changes in accounting principles | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year		$ (12)
Net (charge-offs)/recoveries:
Balance at end of period			$ (12)