BANK LOANS, NET, Changes in the Allowance for Loan Losses (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Changes in the allowance for loan losses [Roll Forward]
Provision for loan losses	$ 109	$ 5	$ 107	$ 21
Loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	216	219	218	203
Provision for loan losses	109	5	107	21
Net (charge-offs)/recoveries:
Charge-offs	0	(6)	0	(6)
Recoveries	0	0	0	1
Net (charge-offs)/recoveries	0	(6)	0	(5)
Foreign exchange translation adjustment	(1)	0	(1)	(1)
Balance at end of period	324	218	324	218
Loans held for investment | C&I loans
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	139	137	139	123
Provision for loan losses	59	6	59	21
Net (charge-offs)/recoveries:
Charge-offs	0	(3)	0	(3)
Recoveries	0	0	0	0
Net (charge-offs)/recoveries	0	(3)	0	(3)
Foreign exchange translation adjustment	(1)	0	(1)	(1)
Balance at end of period	197	140	197	140
Loans held for investment | CRE construction loans
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	2	3	3	3
Provision for loan losses	1	0	0	0
Net (charge-offs)/recoveries:
Charge-offs	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	3	3	3	3
Loans held for investment | CRE loans
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	46	46	46	47
Provision for loan losses	42	2	42	1
Net (charge-offs)/recoveries:
Charge-offs	0	(3)	0	(3)
Recoveries	0	0	0	0
Net (charge-offs)/recoveries	0	(3)	0	(3)
Foreign exchange translation adjustment	0	0	0	0
Balance at end of period	88	45	88	45
Loans held for investment | Tax-exempt loans
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	8	9	9	9
Provision for loan losses	3	(1)	2	(1)
Net (charge-offs)/recoveries:
Charge-offs	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	11	8	11	8
Loans held for investment | Residential mortgage loans
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	17	19	16	17
Provision for loan losses	1	(2)	2	(1)
Net (charge-offs)/recoveries:
Charge-offs	0	0	0	0
Recoveries	0	0	0	1
Net (charge-offs)/recoveries	0	0	0	1
Foreign exchange translation adjustment	0	0	0	0
Balance at end of period	18	17	18	17
Loans held for investment | SBL and other
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	4	5	5	4
Provision for loan losses	3	0	2	1
Net (charge-offs)/recoveries:
Charge-offs	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	$ 7	$ 5	$ 7	$ 5