BANK LOANS, NET, Changes in the Allowance for Loan Losses (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Changes in the allowance for loan losses [Roll Forward]
Provision/(benefit) for loan losses	$ 81	$ (5)	$ 188	$ 16
Net (charge-offs)/recoveries:
Charge-offs related to loan sales	61		61	2
Loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	324	218	218	203
Provision/(benefit) for loan losses	81	(5)	188	16
Net (charge-offs)/recoveries:
Charge-offs (1)	(73)	0	(73)	(6)
Recoveries	1	1	1	2
Net (charge-offs)/recoveries	(72)	1	(72)	(4)
Foreign exchange translation adjustment	1	1	0	0
Balance at end of period	334	215	334	215
Loans held for investment | C&I loans
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	197	140	139	123
Provision/(benefit) for loan losses	59	(8)	118	13
Net (charge-offs)/recoveries:
Charge-offs (1)	(71)	0	(71)	(3)
Recoveries	0	1	0	1
Net (charge-offs)/recoveries	(71)	1	(71)	(2)
Foreign exchange translation adjustment	1	1	0	0
Balance at end of period	186	134	186	134
Loans held for investment | CRE construction loans
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	3	3	3	3
Provision/(benefit) for loan losses	1	1	1	1
Net (charge-offs)/recoveries:
Charge-offs (1)	0	0	0	0
Recoveries	0	0	0	0
Net (charge-offs)/recoveries	0	0	0	0
Foreign exchange translation adjustment	0	0	0	0
Balance at end of period	4	4	4	4
Loans held for investment | CRE loans
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	88	45	46	47
Provision/(benefit) for loan losses	20	1	62	2
Net (charge-offs)/recoveries:
Charge-offs (1)	(2)	0	(2)	(3)
Recoveries	0	0	0	0
Net (charge-offs)/recoveries	(2)	0	(2)	(3)
Foreign exchange translation adjustment	0	0	0	0
Balance at end of period	106	46	106	46
Loans held for investment | Tax-exempt loans
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	11	8	9	9
Provision/(benefit) for loan losses	2	1	4	0
Net (charge-offs)/recoveries:
Charge-offs (1)	0	0	0	0
Recoveries	0	0	0	0
Net (charge-offs)/recoveries	0	0	0	0
Foreign exchange translation adjustment	0	0	0	0
Balance at end of period	13	9	13	9
Loans held for investment | Residential mortgage loans
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	18	17	16	17
Provision/(benefit) for loan losses	1	(1)	3	(2)
Net (charge-offs)/recoveries:
Charge-offs (1)	0	0	0	0
Recoveries	1	0	1	1
Net (charge-offs)/recoveries	1	0	1	1
Foreign exchange translation adjustment	0	0	0	0
Balance at end of period	20	16	20	16
Loans held for investment | SBL and other
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	7	5	5	4
Provision/(benefit) for loan losses	(2)	1	0	2
Net (charge-offs)/recoveries:
Charge-offs (1)	0	0	0	0
Recoveries	0	0	0	0
Net (charge-offs)/recoveries	0	0	0	0
Foreign exchange translation adjustment	0	0	0	0
Balance at end of period	$ 5	$ 6	$ 5	$ 6