BANK LOANS, NET, Changes in the Allowance for Loan Losses (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022	Mar. 31, 2023	Sep. 30, 2022
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period			$ 396
Net (charge-offs)/recoveries:
Balance at end of period	$ 456		456
Increase (decrease) in allowance for credit losses	41		60
Loans held for investment
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	415	$ 328	396	$ 320
Initial allowance on acquired purchased credit deteriorated (“PCD”) loans		3		3
Total provision/(benefit) for credit losses	54	56	96	66
Net (charge-offs)/recoveries:
Charge-offs	(15)	(15)	(40)	(18)
Recoveries	0	5	3	6
Net (charge-offs)/recoveries	(15)	(10)	(37)	(12)
Foreign exchange translation adjustment	2	0	1	0
Balance at end of period	$ 456	$ 377	$ 456	$ 377
ACL by loan portfolio segment as a % of total ACL	100.00%	100.00%	100.00%	100.00%
Loans held for investment | Initial provision for credit losses on non-PCD loans acquired with TriState Capital
Changes in the allowance for loan losses [Roll Forward]
Total provision/(benefit) for credit losses		$ 26		$ 26
Loans held for investment | Provision/(benefit) for credit losses
Changes in the allowance for loan losses [Roll Forward]
Total provision/(benefit) for credit losses		30		40
Loans held for investment | SBL
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	$ 5	3	$ 3	4
Initial allowance on acquired purchased credit deteriorated (“PCD”) loans		0		0
Total provision/(benefit) for credit losses	0	1	2	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	$ 5	$ 4	$ 5	$ 4
ACL by loan portfolio segment as a % of total ACL	1.10%	1.10%	1.10%	1.10%
Loans held for investment | SBL | Initial provision for credit losses on non-PCD loans acquired with TriState Capital
Changes in the allowance for loan losses [Roll Forward]
Total provision/(benefit) for credit losses		$ 2		$ 2
Loans held for investment | SBL | Provision/(benefit) for credit losses
Changes in the allowance for loan losses [Roll Forward]
Total provision/(benefit) for credit losses		(1)		(2)
Loans held for investment | C&I loans
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	$ 219	195	$ 226	191
Initial allowance on acquired purchased credit deteriorated (“PCD”) loans		1		1
Total provision/(benefit) for credit losses	(8)	22	10	29
Net (charge-offs)/recoveries:
Charge-offs	(6)	(11)	(30)	(14)
Recoveries	0	0	0	0
Net (charge-offs)/recoveries	(6)	(11)	(30)	(14)
Foreign exchange translation adjustment	1	0	0	0
Balance at end of period	$ 206	$ 207	$ 206	$ 207
ACL by loan portfolio segment as a % of total ACL	45.30%	54.90%	45.30%	54.90%
Loans held for investment | C&I loans | Initial provision for credit losses on non-PCD loans acquired with TriState Capital
Changes in the allowance for loan losses [Roll Forward]
Total provision/(benefit) for credit losses		$ 5		$ 5
Loans held for investment | C&I loans | Provision/(benefit) for credit losses
Changes in the allowance for loan losses [Roll Forward]
Total provision/(benefit) for credit losses		17		24
Loans held for investment | CRE loans
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	$ 100	71	$ 87	66
Initial allowance on acquired purchased credit deteriorated (“PCD”) loans		2		2
Total provision/(benefit) for credit losses	55	19	66	24
Net (charge-offs)/recoveries:
Charge-offs	(9)	(4)	(10)	(4)
Recoveries	0	5	3	5
Net (charge-offs)/recoveries	(9)	1	(7)	1
Foreign exchange translation adjustment	1	0	1	0
Balance at end of period	$ 147	$ 93	$ 147	$ 93
ACL by loan portfolio segment as a % of total ACL	32.20%	24.70%	32.20%	24.70%
Loans held for investment | CRE loans | Initial provision for credit losses on non-PCD loans acquired with TriState Capital
Changes in the allowance for loan losses [Roll Forward]
Total provision/(benefit) for credit losses		$ 19		$ 19
Loans held for investment | CRE loans | Provision/(benefit) for credit losses
Changes in the allowance for loan losses [Roll Forward]
Total provision/(benefit) for credit losses		0		5
Loans held for investment | REIT loans
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	$ 15	25	$ 21	22
Initial allowance on acquired purchased credit deteriorated (“PCD”) loans		0		0
Total provision/(benefit) for credit losses	1	(2)	(5)	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	$ 16	$ 23	$ 16	$ 23
ACL by loan portfolio segment as a % of total ACL	3.50%	6.10%	3.50%	6.10%
Loans held for investment | REIT loans | Initial provision for credit losses on non-PCD loans acquired with TriState Capital
Changes in the allowance for loan losses [Roll Forward]
Total provision/(benefit) for credit losses		$ 0		$ 0
Loans held for investment | REIT loans | Provision/(benefit) for credit losses
Changes in the allowance for loan losses [Roll Forward]
Total provision/(benefit) for credit losses		(2)		1
Loans held for investment | Residential mortgage loans
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	$ 74	32	$ 57	35
Initial allowance on acquired purchased credit deteriorated (“PCD”) loans		0		0
Total provision/(benefit) for credit losses	6	16	23	12
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	$ 80	$ 48	$ 80	$ 48
ACL by loan portfolio segment as a % of total ACL	17.50%	12.70%	17.50%	12.70%
Loans held for investment | Residential mortgage loans | Initial provision for credit losses on non-PCD loans acquired with TriState Capital
Changes in the allowance for loan losses [Roll Forward]
Total provision/(benefit) for credit losses		$ 0		$ 0
Loans held for investment | Residential mortgage loans | Provision/(benefit) for credit losses
Changes in the allowance for loan losses [Roll Forward]
Total provision/(benefit) for credit losses		16		12
Loans held for investment | Tax-exempt loans
Changes in the allowance for loan losses [Roll Forward]
Balance at beginning of period	$ 2	2	$ 2	2
Initial allowance on acquired purchased credit deteriorated (“PCD”) loans		0		0
Total 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
ACL by loan portfolio segment as a % of total ACL	0.40%	0.50%	0.40%	0.50%
Loans held for investment | Tax-exempt loans | Initial provision for credit losses on non-PCD loans acquired with TriState Capital
Changes in the allowance for loan losses [Roll Forward]
Total provision/(benefit) for credit losses		$ 0		$ 0
Loans held for investment | Tax-exempt loans | Provision/(benefit) for credit losses
Changes in the allowance for loan losses [Roll Forward]
Total provision/(benefit) for credit losses		$ 0		$ 0
Unfunded lending commitments
Net (charge-offs)/recoveries:
Unfunded lending commitments	$ 28		$ 28		$ 21	$ 19