BANK LOANS, NET, Allowance for Loan Losses (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Changes in the allowance for loan losses [Roll Forward]
Provision for loan losses	$ 233	$ 22	$ 20
Net (charge-offs)/recoveries:
Loans sold	87	2
Loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year	218	203
Provision for loan losses	233	22
Net (charge-offs)/recoveries:
Charge-offs	(100)	(8)
Recoveries	2	2
Net (charge-offs)/recoveries	(98)	(6)
Foreign exchange translation adjustment	1	(1)
Balance at end of period	354	218	203
C&I | Loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year	139	123
Provision for loan losses	157	19
Net (charge-offs)/recoveries:
Charge-offs	(96)	(2)
Recoveries	0	0
Net (charge-offs)/recoveries	(96)	(2)
Foreign exchange translation adjustment	0	(1)
Balance at end of period	200	139	123
CRE construction | Loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year	3	3
Provision for loan losses	0	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	3	3	3
CRE loans | Loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year	46	47
Provision for loan losses	71	4
Net (charge-offs)/recoveries:
Charge-offs	(4)	(5)
Recoveries	0	0
Net (charge-offs)/recoveries	(4)	(5)
Foreign exchange translation adjustment	1	0
Balance at end of period	114	46	47
Tax-exempt | Loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year	9	9
Provision for loan losses	5	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	14	9	9
Residential mortgage loans | Loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year	16	17
Provision for loan losses	0	(2)
Net (charge-offs)/recoveries:
Charge-offs	0	(1)
Recoveries	2	2
Net (charge-offs)/recoveries	2	1
Foreign exchange translation adjustment	0	0
Balance at end of period	18	16	17
SBL and other | Loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of year	5	4
Provision for loan losses	0	1
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	$ 5	$ 5	$ 4