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Loans Receivable - Schedule of Loans Receivable (Details) - USD ($) $ in Thousands	Jun. 30, 2026	Mar. 31, 2026	Sep. 30, 2025	Jun. 30, 2025	Mar. 31, 2025	Sep. 30, 2024
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 21,633,392		$ 21,274,392
Ratio of loan type to all loans	100.00%		100.00%
Allowance for credit losses on loans	$ 214,831	$ 201,950	$ 199,720	$ 198,768	$ 202,709	$ 203,753
Loans in process	1,186,436		773,606
Net deferred fees, costs and discounts	214,249		212,448
Total loan contra accounts	1,615,516		1,185,774
Net loans	20,017,876		20,088,618
Commercial loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 13,837,593		$ 12,651,754
Ratio of loan type to all loans	64.00%		59.50%
Allowance for credit losses on loans	$ 177,866	160,492	$ 152,134	149,374	151,281	153,373
Commercial loans \| Multi-family
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 4,721,719		$ 4,718,480
Ratio of loan type to all loans	21.80%		22.20%
Allowance for credit losses on loans	$ 25,646	25,923	$ 25,953	26,806	26,556	25,248
Commercial loans \| Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 3,642,259		$ 3,604,600
Ratio of loan type to all loans	16.90%		16.90%
Allowance for credit losses on loans	$ 40,772	41,503	$ 41,988	42,275	38,469	39,210
Commercial loans \| Commercial & industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 3,110,005		$ 2,392,685
Ratio of loan type to all loans	14.40%		11.30%
Allowance for credit losses on loans	$ 84,893	68,662	$ 59,163	56,773	60,147	58,748
Commercial loans \| Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 2,144,343		$ 1,756,890
Ratio of loan type to all loans	9.90%		8.30%
Allowance for credit losses on loans	$ 18,095	16,712	$ 18,136	17,113	19,406	22,267
Commercial loans \| Land - acquisition & development
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 219,267		$ 179,099
Ratio of loan type to all loans	1.00%		0.80%
Allowance for credit losses on loans	$ 8,460	7,692	$ 6,894	6,407	6,703	7,900
Consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 7,795,799		$ 8,622,638
Ratio of loan type to all loans	36.00%		40.50%
Allowance for credit losses on loans	$ 36,965	41,458	$ 47,586	49,394	51,428	50,380
Consumer loans \| Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 35,260		$ 150,237
Ratio of loan type to all loans	0.20%		0.70%
Allowance for credit losses on loans	$ 128	302	$ 610	746	1,076	1,427
Consumer loans \| Single-family residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 7,389,117		$ 8,053,771
Ratio of loan type to all loans	34.20%		37.80%
Allowance for credit losses on loans	$ 29,504	33,679	$ 38,880	39,998	40,963	40,523
Consumer loans \| Land - consumer lot loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 72,451		$ 89,298
Ratio of loan type to all loans	0.30%		0.40%
Allowance for credit losses on loans	$ 1,707	1,821	$ 2,105	2,277	2,415	2,564
Consumer loans \| HELOC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 244,726		$ 267,871
Ratio of loan type to all loans	1.10%		1.30%
Allowance for credit losses on loans	$ 2,764	2,850	$ 3,069	3,122	3,257	3,049
Consumer loans \| Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 54,245		$ 61,461
Ratio of loan type to all loans	0.20%		0.30%
Allowance for credit losses on loans	$ 2,862	$ 2,806	$ 2,922	$ 3,251	$ 3,717	$ 2,817