BANK LOANS, NET, Changes in the Allowance for Loan Losses (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Sep. 30, 2020
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period			$ 354
Net (charge-offs)/recoveries:
Balance at end of period	$ 345		345
Unfunded lending commitments	17		17		$ 20	$ 12
Cumulative Effect, Period of Adoption, Adjustment
Net (charge-offs)/recoveries:
Balance at end of period	33		33
Unfunded lending commitments						$ 8
Loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	378	$ 216	354	$ 218
Provision/(benefit) for credit losses	(32)	109	(18)	107
Net (charge-offs)/recoveries:
Charge-offs	(2)	0	(2)	0
Recoveries	0	0	0	0
Net (charge-offs)/recoveries	(2)	0	(2)	0
Foreign exchange translation adjustment	1	(1)	2	(1)
Balance at end of period	345	324	345	324
Loans held for investment | Cumulative Effect, Period of Adoption, Adjustment
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	198	139	200	139
Provision/(benefit) for credit losses	7	58	(15)	58
Net (charge-offs)/recoveries:
Charge-offs	(2)	0	(2)	0
Recoveries	0	0	0	0
Net (charge-offs)/recoveries	(2)	0	(2)	0
Foreign exchange translation adjustment	0	(1)	1	(1)
Balance at end of period	203	196	203	196
Loans held for investment | C&I loans | Cumulative Effect, Period of Adoption, Adjustment
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	112	36	81	34
Provision/(benefit) for credit losses	(39)	18	3	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	1	0	1	0
Balance at end of period	74	54	74	54
Loans held for investment | CRE loans | Cumulative Effect, Period of Adoption, Adjustment
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	30	12	36	15
Provision/(benefit) for credit losses	6	26	9	23
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	36	38	36	38
Loans held for investment | REIT loans | Cumulative Effect, Period of Adoption, Adjustment
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	8	14	9
Provision/(benefit) for credit losses	0	3	0	2
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	11	2	11
Loans held for investment | Tax-exempt loans | Cumulative Effect, Period of Adoption, Adjustment
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	33	17	18	16
Provision/(benefit) for credit losses	(7)	1	(16)	2
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	18	26	18
Loans held for investment | Residential mortgage loans | Cumulative Effect, Period of Adoption, Adjustment
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	4	5	5
Provision/(benefit) for credit losses	1	3	1	2
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	$ 7	4	$ 7
Loans held for investment | SBL and other | Cumulative Effect, Period of Adoption, Adjustment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period			$ (2)