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Allowance for Loan Losses - Loans Based on Credit Quality Indicators (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	$ 21,120,107	$ 17,653,757
Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	$ 20,615,520	$ 17,246,669
Pass | Accounts Receivable | Credit Concentration Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Total grade as a % of total loans	97.60%	97.70%
Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	$ 77,159	$ 101,922
Special Mention | Accounts Receivable | Credit Concentration Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Total grade as a % of total loans	0.40%	0.60%
Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	$ 427,428	$ 305,166
Substandard | Accounts Receivable | Credit Concentration Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Total grade as a % of total loans	2.00%	1.70%
Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	$ 0	$ 0
Doubtful | Accounts Receivable | Credit Concentration Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Total grade as a % of total loans	0.00%	0.00%
Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	$ 0	$ 0
Loss | Accounts Receivable | Credit Concentration Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Total grade as a % of total loans	0.00%	0.00%
Commercial loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	$ 12,205,420	$ 10,507,610
Commercial loans | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	11,723,996	10,119,666
Commercial loans | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	77,159	101,922
Commercial loans | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	404,265	286,022
Commercial loans | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	0	0
Commercial loans | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	0	0
Commercial loans | Multi-family
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	4,556,200	2,886,594
Commercial loans | Multi-family | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	4,476,283	2,772,538
Commercial loans | Multi-family | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	12,377	90,428
Commercial loans | Multi-family | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	67,540	23,628
Commercial loans | Multi-family | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	0	0
Commercial loans | Multi-family | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	0	0
Commercial loans | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	3,732,155	3,310,101
Commercial loans | Commercial real estate | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	3,587,682	3,207,596
Commercial loans | Commercial real estate | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	40,297	4,847
Commercial loans | Commercial real estate | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	104,176	97,658
Commercial loans | Commercial real estate | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	0	0
Commercial loans | Commercial real estate | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	0	0
Commercial loans | Commercial & industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	2,332,732	2,315,318
Commercial loans | Commercial & industrial | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	2,107,780	2,164,876
Commercial loans | Commercial & industrial | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	21,264	6,647
Commercial loans | Commercial & industrial | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	203,688	143,795
Commercial loans | Commercial & industrial | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	0	0
Commercial loans | Commercial & industrial | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	0	0
Commercial loans | Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	1,424,016	1,838,936
Commercial loans | Construction | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	1,392,332	1,818,266
Commercial loans | Construction | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	3,221	0
Commercial loans | Construction | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	28,463	20,670
Commercial loans | Construction | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	0	0
Commercial loans | Construction | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	0	0
Commercial loans | Land - acquisition & development
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	160,317	156,661
Commercial loans | Land - acquisition & development | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	159,919	156,390
Commercial loans | Land - acquisition & development | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	0	0
Commercial loans | Land - acquisition & development | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	398	271
Commercial loans | Land - acquisition & development | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	0	0
Commercial loans | Land - acquisition & development | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	0	0
Consumer loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	8,914,687	7,146,147
Consumer loans | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	8,891,524	7,127,003
Consumer loans | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	0	0
Consumer loans | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	23,163	19,144
Consumer loans | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	0	0
Consumer loans | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	0	0
Consumer loans | Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	182,415	324,451
Consumer loans | Construction | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	181,567	324,363
Consumer loans | Construction | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	0	0
Consumer loans | Construction | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	848	88
Consumer loans | Construction | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	0	0
Consumer loans | Construction | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	0	0
Consumer loans | Single-family residential
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	8,280,300	6,388,990
Consumer loans | Single-family residential | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	8,258,812	6,370,936
Consumer loans | Single-family residential | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	0	0
Consumer loans | Single-family residential | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	21,488	18,054
Consumer loans | Single-family residential | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	0	0
Consumer loans | Single-family residential | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	0	0
Consumer loans | Land - consumer lot loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	108,060	124,842
Consumer loans | Land - consumer lot loans | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	108,060	124,588
Consumer loans | Land - consumer lot loans | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	0	0
Consumer loans | Land - consumer lot loans | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	0	254
Consumer loans | Land - consumer lot loans | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	0	0
Consumer loans | Land - consumer lot loans | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	0	0
Consumer loans | HELOC
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	269,857	237,754
Consumer loans | HELOC | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	269,261	237,018
Consumer loans | HELOC | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	0	0
Consumer loans | HELOC | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	596	736
Consumer loans | HELOC | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	0	0
Consumer loans | HELOC | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	0	0
Consumer loans | Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	74,055	70,110
Consumer loans | Consumer | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	73,824	70,098
Consumer loans | Consumer | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	0	0
Consumer loans | Consumer | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	231	12
Consumer loans | Consumer | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	0	0
Consumer loans | Consumer | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	$ 0	$ 0

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