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Allowance for Losses on Loans - Internally Assigned Grade (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Mar. 31, 2025	Sep. 30, 2024
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	$ 21,122,710	$ 21,120,107
Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	$ 20,427,619	$ 20,615,520
Pass | Accounts Receivable | Credit Concentration Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Total grade as a % of total loans	96.71%	97.61%
Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	$ 165,613	$ 77,159
Special mention | Accounts Receivable | Credit Concentration Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Total grade as a % of total loans	0.78%	0.37%
Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	$ 524,156	$ 427,428
Substandard | Accounts Receivable | Credit Concentration Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Total grade as a % of total loans	2.48%	2.02%
Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	$ 5,322	$ 0
Doubtful | Accounts Receivable | Credit Concentration Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Total grade as a % of total loans	0.03%	0.00%
Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Gross 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]
Gross loans	$ 12,238,959	$ 12,205,420
Commercial loans | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	11,563,915	11,723,996
Commercial loans | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	165,613	77,159
Commercial loans | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	504,109	404,265
Commercial loans | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	5,322	0
Commercial loans | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Commercial loans | Multi-family
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	4,867,340	4,556,200
Commercial loans | Multi-family | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	4,677,592	4,476,283
Commercial loans | Multi-family | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	110,502	12,377
Commercial loans | Multi-family | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	76,803	67,540
Commercial loans | Multi-family | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	2,443	0
Commercial loans | Multi-family | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Commercial loans | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	3,639,477	3,732,155
Commercial loans | Commercial real estate | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	3,507,589	3,587,682
Commercial loans | Commercial real estate | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	6,478	40,297
Commercial loans | Commercial real estate | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	122,531	104,176
Commercial loans | Commercial real estate | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	2,879	0
Commercial loans | Commercial real estate | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Commercial loans | Commercial & industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	2,384,745	2,332,732
Commercial loans | Commercial & industrial | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	2,097,185	2,107,780
Commercial loans | Commercial & industrial | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	44,439	21,264
Commercial loans | Commercial & industrial | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	243,121	203,688
Commercial loans | Commercial & industrial | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Commercial loans | Commercial & industrial | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Commercial loans | Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	1,211,336	1,424,016
Commercial loans | Construction | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	1,146,257	1,392,332
Commercial loans | Construction | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	4,194	3,221
Commercial loans | Construction | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	60,885	28,463
Commercial loans | Construction | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Commercial loans | Construction | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Commercial loans | Land - acquisition & development
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	136,061	160,317
Commercial loans | Land - acquisition & development | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	135,292	159,919
Commercial loans | Land - acquisition & development | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Commercial loans | Land - acquisition & development | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	769	398
Commercial loans | Land - acquisition & development | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Commercial loans | Land - acquisition & development | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	8,883,751	8,914,687
Consumer loans | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	8,863,704	8,891,524
Consumer loans | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	20,047	23,163
Consumer loans | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans | Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	137,501	182,415
Consumer loans | Construction | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	136,653	181,567
Consumer loans | Construction | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans | Construction | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	848	848
Consumer loans | Construction | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans | Construction | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans | Single-family residential
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	8,264,318	8,280,300
Consumer loans | Single-family residential | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	8,245,584	8,258,812
Consumer loans | Single-family residential | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans | Single-family residential | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	18,734	21,488
Consumer loans | Single-family residential | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans | Single-family residential | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans | Land - consumer lot loans
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	101,784	108,060
Consumer loans | Land - consumer lot loans | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	101,776	108,060
Consumer loans | Land - consumer lot loans | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans | Land - consumer lot loans | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	8	0
Consumer loans | Land - consumer lot loans | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans | Land - consumer lot loans | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans | HELOC
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	288,228	269,857
Consumer loans | HELOC | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	287,928	269,261
Consumer loans | HELOC | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans | HELOC | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	300	596
Consumer loans | HELOC | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans | HELOC | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans | Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	91,920	74,055
Consumer loans | Consumer | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	91,763	73,824
Consumer loans | Consumer | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans | Consumer | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	157	231
Consumer loans | Consumer | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans | Consumer | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	$ 0	$ 0

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