BANK LOANS, NET, Changes in the Allowance for Loan Losses (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Mar. 31, 2021	Sep. 30, 2020
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period			$ 354
Net (charge-offs)/recoveries:
Decrease in allowance for credit losses	$ 23
Unfunded lending commitments	15		15		$ 17	$ 12
Impact of CECL adoption
Net (charge-offs)/recoveries:
Unfunded lending commitments						$ 8
Loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	345	$ 324	354	$ 218
Provision/(benefit) for credit losses	(19)	81	(37)	188
Net (charge-offs)/recoveries:
Charge-offs	(4)	(73)	(6)	(73)
Recoveries	0	1	0	1
Net (charge-offs)/recoveries	(4)	(72)	(6)	(72)
Foreign exchange translation adjustment	0	1	2	0
Balance at end of period	322	334	322	334
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	203	196	200	139
Provision/(benefit) for credit losses	(14)	59	(29)	117
Net (charge-offs)/recoveries:
Charge-offs	(1)	(71)	(3)	(71)
Recoveries	0	0	0	0
Net (charge-offs)/recoveries	(1)	(71)	(3)	(71)
Foreign exchange translation adjustment	0	1	1	0
Balance at end of period	188	185	188	185
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	74	54	81	34
Provision/(benefit) for credit losses	2	24	5	44
Net (charge-offs)/recoveries:
Charge-offs	(3)	(2)	(3)	(2)
Recoveries	0	0	0	0
Net (charge-offs)/recoveries	(3)	(2)	(3)	(2)
Foreign exchange translation adjustment	0	0	1	0
Balance at end of period	73	76	73	76
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	36	38	36	15
Provision/(benefit) for credit losses	(10)	(3)	(1)	20
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	26	35	26	35
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	11	14	9
Provision/(benefit) for credit losses	0	2	0	4
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	13	2	13
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	26	18	18	16
Provision/(benefit) for credit losses	3	1	(13)	3
Net (charge-offs)/recoveries:
Charge-offs	0	0	0	0
Recoveries	0	1	0	1
Net (charge-offs)/recoveries	0	1	0	1
Foreign exchange translation adjustment	0	0	0	0
Balance at end of period	29	20	29	20
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	4	7	5	5
Provision/(benefit) for credit losses	0	(2)	1	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	$ 4	$ 5	4	$ 5
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)