BANK LOANS, NET, Allowance for Loan Losses (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year	$ 354	$ 218	$ 203
Provision/(benefit) for credit losses	(32)	233	22
Net (charge-offs)/recoveries:
Balance at end of period	320	354	218
Increase (decrease) in allowance for credit losses	43
Reserve for unfunded lending commitments	13	12
Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year	354	218
Provision/(benefit) for credit losses	(32)	233
Net (charge-offs)/recoveries:
Charge-offs	(14)	(100)
Recoveries	1	2
Net (charge-offs)/recoveries	(13)	(98)
Foreign exchange translation adjustment	2	1
Balance at end of period	320	354	218
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
Net (charge-offs)/recoveries:
Balance at end of period		9
Unfunded lending commitments | Cumulative adjustments for changes in accounting principles
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year	8
Net (charge-offs)/recoveries:
Balance at end of period		8
C&I | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year	200	139
Provision/(benefit) for credit losses	(25)	157
Net (charge-offs)/recoveries:
Charge-offs	(4)	(96)
Recoveries	0	0
Net (charge-offs)/recoveries	(4)	(96)
Foreign exchange translation adjustment	1	0
Balance at end of period	191	200	139
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
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	81	34
Provision/(benefit) for credit losses	5	48
Net (charge-offs)/recoveries:
Charge-offs	(10)	(2)
Recoveries	0	0
Net (charge-offs)/recoveries	(10)	(2)
Foreign exchange translation adjustment	1	1
Balance at end of period	66	81	34
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)
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	36	15
Provision/(benefit) for credit losses	(5)	23
Net (charge-offs)/recoveries:
Charge-offs	0	(2)
Recoveries	0	0
Net (charge-offs)/recoveries	0	(2)
Foreign exchange translation adjustment	0	0
Balance at end of period	22	36	15
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)
Net (charge-offs)/recoveries:
Balance at end of period		(9)
Tax-exempt | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year	14	9
Provision/(benefit) for credit losses	0	5
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	14	9
Tax-exempt | 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)
Net (charge-offs)/recoveries:
Balance at end of period		(12)
Residential mortgage loans | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year	18	16
Provision/(benefit) for credit losses	(8)	0
Net (charge-offs)/recoveries:
Charge-offs	0	0
Recoveries	1	2
Net (charge-offs)/recoveries	1	2
Foreign exchange translation adjustment	0	0
Balance at end of period	35	18	16
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
Net (charge-offs)/recoveries:
Balance at end of period		24
SBL and other | Total loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year	5	5
Provision/(benefit) for credit losses	1	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	4	5	$ 5
SBL and other | 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)
Net (charge-offs)/recoveries:
Balance at end of period		$ (2)