BANK LOANS, NET, Changes in the Allowance for Loan Losses (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Mar. 31, 2022	Mar. 31, 2021	Dec. 31, 2021	Sep. 30, 2021
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period			$ 320
Net (charge-offs)/recoveries:
Balance at end of period	$ 328		328
Decrease in allowance for credit losses	20		8
Unfunded lending commitments	12		12		$ 12	$ 13
Loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	308	$ 378	320	$ 354
Provision/(benefit) for credit losses	21	(32)	10	(18)
Net (charge-offs)/recoveries:
Charge-offs	(1)	(2)	(3)	(2)
Recoveries	0	0	1	0
Net (charge-offs)/recoveries	(1)	(2)	(2)	(2)
Foreign exchange translation adjustment	0	1	0	2
Balance at end of period	328	345	328	345
Loans held for investment | Impact of CECL adoption
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period				9
Loans held for investment | C&I loans
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	179	198	191	200
Provision/(benefit) for credit losses	17	7	7	(15)
Net (charge-offs)/recoveries:
Charge-offs	(1)	(2)	(3)	(2)
Recoveries	0	0	0	0
Net (charge-offs)/recoveries	(1)	(2)	(3)	(2)
Foreign exchange translation adjustment	0	0	0	1
Balance at end of period	195	203	195	203
Loans held for investment | C&I loans | Impact of CECL adoption
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period				19
Loans held for investment | CRE loans
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	72	112	66	81
Provision/(benefit) for credit losses	(1)	(39)	5	3
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	1	0	1
Balance at end of period	71	74	71	74
Loans held for investment | CRE loans | Impact of CECL adoption
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period				(11)
Loans held for investment | REIT loans
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	22	30	22	36
Provision/(benefit) for credit losses	3	6	3	9
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	25	36	25	36
Loans held for investment | REIT loans | Impact of CECL adoption
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period				(9)
Loans held for investment | Tax-exempt loans
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	2	2	2	14
Provision/(benefit) for credit losses	0	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	2	2	2	2
Loans held for investment | Tax-exempt loans | Impact of CECL adoption
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period				(12)
Loans held for investment | Residential mortgage loans
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	30	33	35	18
Provision/(benefit) for credit losses	2	(7)	(4)	(16)
Net (charge-offs)/recoveries:
Charge-offs	0	0	0	0
Recoveries	0	0	1	0
Net (charge-offs)/recoveries	0	0	1	0
Foreign exchange translation adjustment	0	0	0	0
Balance at end of period	32	26	32	26
Loans held for investment | Residential mortgage loans | Impact of CECL adoption
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period				24
Loans held for investment | SBL and other
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	3	3	4	5
Provision/(benefit) for credit losses	0	1	(1)	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	$ 3	$ 4	$ 3	4
Loans held for investment | SBL and other | Impact of CECL adoption
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period				$ (2)