Allowance for Losses on Loans - Allowance for Loan Losses (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	$ 201,577	$ 177,420	$ 177,207	$ 172,808
Charge-offs	(2,713)	(10,543)	(3,351)	(16,897)
Recoveries	1,460	192	2,065	1,158
Provision & Transfers	3,500	11,000	27,903	21,000
Ending Allowance	203,824	178,069	203,824	178,069
Provision and transfers	3,500	11,000	27,903	21,000
Non-PCD
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Provision & Transfers			16,000
Provision and transfers			16,000
PCD | Luther Burbank
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Provision & Transfers			7,403
Provision and transfers			7,403
Unfunded commitments
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Provision & Transfers	2,000	2,000	3,000	6,000
Provision and transfers	2,000	2,000	3,000	6,000
Commercial loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	149,413	137,665	137,194	133,476
Charge-offs	(2,549)	(10,469)	(2,694)	(16,605)
Recoveries	1,022	34	1,159	143
Provision & Transfers	5,309	10,955	17,536	21,171
Ending Allowance	153,195	138,185	153,195	138,185
Provision and transfers	5,309	10,955	17,536	21,171
Commercial loans | Multi-family
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	21,979	13,038	13,155	12,013
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision & Transfers	3,220	(62)	12,044	963
Ending Allowance	25,199	12,976	25,199	12,976
Provision and transfers	3,220	(62)	12,044	963
Commercial loans | Commercial real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	32,991	27,803	28,842	25,814
Charge-offs	(203)	0	(203)	0
Recoveries	2	0	4	5
Provision & Transfers	6,966	329	11,113	2,313
Ending Allowance	39,756	28,132	39,756	28,132
Provision and transfers	6,966	329	11,113	2,313
Commercial loans | Commercial & industrial
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	59,261	63,301	58,773	57,210
Charge-offs	(2,346)	(10,469)	(2,473)	(16,605)
Recoveries	1,003	10	1,067	84
Provision & Transfers	(85)	9,073	466	21,226
Ending Allowance	57,833	61,915	57,833	61,915
Provision and transfers	(85)	9,073	466	21,226
Commercial loans | Construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	27,317	26,027	29,408	26,161
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision & Transfers	(4,610)	1,889	(6,701)	1,755
Ending Allowance	22,707	27,916	22,707	27,916
Provision and transfers	(4,610)	1,889	(6,701)	1,755
Commercial loans | Land - acquisition & development
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	7,865	7,496	7,016	12,278
Charge-offs	0	0	(18)	0
Recoveries	17	24	88	54
Provision & Transfers	(182)	(274)	614	(5,086)
Ending Allowance	7,700	7,246	7,700	7,246
Provision and transfers	(182)	(274)	614	(5,086)
Consumer loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	52,164	39,755	40,013	39,332
Charge-offs	(164)	(74)	(657)	(292)
Recoveries	438	158	906	1,015
Provision & Transfers	(1,809)	45	10,367	(171)
Ending Allowance	50,629	39,884	50,629	39,884
Provision and transfers	(1,809)	45	10,367	(171)
Consumer loans | Construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	1,918	3,456	2,781	3,410
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision & Transfers	(424)	(235)	(1,287)	(189)
Ending Allowance	1,494	3,221	1,494	3,221
Provision and transfers	(424)	(235)	(1,287)	(189)
Consumer loans | Single-family residential
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	41,054	26,916	28,029	25,518
Charge-offs	(13)	0	(144)	(34)
Recoveries	118	18	293	552
Provision & Transfers	(843)	149	12,138	1,047
Ending Allowance	40,316	27,083	40,316	27,083
Provision and transfers	(843)	149	12,138	1,047
Consumer loans | Land - consumer lot loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	3,214	3,945	3,512	5,047
Charge-offs	0	0	0	0
Recoveries	2	9	57	14
Provision & Transfers	(568)	(184)	(921)	(1,291)
Ending Allowance	2,648	3,770	2,648	3,770
Provision and transfers	(568)	(184)	(921)	(1,291)
Consumer loans | HELOC
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	2,974	2,662	2,859	2,482
Charge-offs	0	0	0	0
Recoveries	1	0	3	1
Provision & Transfers	(51)	81	62	260
Ending Allowance	2,924	2,743	2,924	2,743
Provision and transfers	(51)	81	62	260
Consumer loans | Consumer
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	3,004	2,776	2,832	2,875
Charge-offs	(151)	(74)	(513)	(258)
Recoveries	317	131	553	448
Provision & Transfers	77	234	375	2
Ending Allowance	3,247	3,067	3,247	3,067
Provision and transfers	$ 77	$ 234	$ 375	$ 2