Allowance for Losses on Loans - Allowance for Loan Losses (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended	12 Months Ended
Mar. 31, 2025	Mar. 31, 2024	Mar. 31, 2025	Mar. 31, 2024	Sep. 30, 2024
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	$ 204,522	$ 179,320	$ 203,753	$ 177,207	$ 177,207
Charge-offs	(5,443)	(345)	(6,228)	(638)
Recoveries	380	199	934	605
Provision & transfers	3,250	22,403	4,250	24,403
Ending Allowance	202,709	201,577	202,709	201,577	203,753
Financial Asset Acquired and No Credit Deterioration
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Provision & transfers		16,000
Financial Asset Acquired with Credit Deterioration
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Provision & transfers		7,403		7,403
Commercial loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	152,468	139,155	153,373	137,194	137,194
Charge-offs	(4,628)	(65)	(5,148)	(145)	(2,963)
Recoveries	215	53	230	137
Provision & transfers	3,226	10,270	2,826	12,227
Ending Allowance	151,281	149,413	151,281	149,413	153,373
Commercial loans \| Multi-family
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	25,997	13,791	25,248	13,155	13,155
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision & transfers	559	8,188	1,308	8,824
Ending Allowance	26,556	21,979	26,556	21,979	25,248
Commercial loans \| Commercial real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	37,873	29,007	39,210	28,842	28,842
Charge-offs	(4,392)	0	(4,555)	0	(203)
Recoveries	169	0	169	2
Provision & transfers	4,819	3,984	3,645	4,147
Ending Allowance	38,469	32,991	38,469	32,991	39,210
Commercial loans \| Commercial & industrial
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	60,474	60,836	58,748	58,773	58,773
Charge-offs	(236)	(65)	(593)	(127)	(2,611)
Recoveries	38	32	42	64
Provision & transfers	(129)	(1,542)	1,950	551
Ending Allowance	60,147	59,261	60,147	59,261	58,748
Commercial loans \| Construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	20,903	28,863	22,267	29,408	29,408
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision & transfers	(1,497)	(1,546)	(2,861)	(2,091)
Ending Allowance	19,406	27,317	19,406	27,317	22,267
Commercial loans \| Land - acquisition & development
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	7,221	6,658	7,900	7,016	7,016
Charge-offs	0	0	0	(18)	(149)
Recoveries	8	21	19	71
Provision & transfers	(526)	1,186	(1,216)	796
Ending Allowance	6,703	7,865	6,703	7,865	7,900
Consumer loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	52,054	40,165	50,380	40,013	40,013
Charge-offs	(815)	(280)	(1,080)	(493)	(662)
Recoveries	165	146	704	468
Provision & transfers	24	12,133	1,424	12,176
Ending Allowance	51,428	52,164	51,428	52,164	50,380
Consumer loans \| Construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	1,219	2,262	1,427	2,781	2,781
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision & transfers	(143)	(344)	(351)	(863)
Ending Allowance	1,076	1,918	1,076	1,918	1,427
Consumer loans \| Single-family residential
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	42,117	28,556	40,523	28,029	28,029
Charge-offs	(338)	(131)	(338)	(131)	(144)
Recoveries	7	55	463	175
Provision & transfers	(823)	12,574	315	12,981
Ending Allowance	40,963	41,054	40,963	41,054	40,523
Consumer loans \| Land - consumer lot loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	2,527	3,345	2,564	3,512	3,512
Charge-offs	0	0	0	0
Recoveries	0	46	0	55
Provision & transfers	(112)	(177)	(149)	(353)
Ending Allowance	2,415	3,214	2,415	3,214	2,564
Consumer loans \| HELOC
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	3,158	2,973	3,049	2,859	2,859
Charge-offs	0	0	0	0
Recoveries	0	1	2	2
Provision & transfers	99	0	206	113
Ending Allowance	3,257	2,974	3,257	2,974	3,049
Consumer loans \| Consumer
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	3,033	3,029	2,817	2,832	2,832
Charge-offs	(477)	(149)	(742)	(362)	(518)
Recoveries	158	44	239	236
Provision & transfers	1,003	80	1,403	298
Ending Allowance	3,717	3,004	3,717	3,004	$ 2,817
Unfunded commitments
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Provision & transfers	$ 500	$ 1,000	$ 1,500	$ 1,000

Amount excluding accrued interest, of allowance for credit loss on financing receivable. Excludes net investment in lease.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479344/326-20-45-1

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+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (e)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

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+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-6

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

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