Loans Receivable - Schedule of Loans Receivable (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Mar. 31, 2024	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Sep. 30, 2022
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for sale	$ 468,527		$ 0
Gross loans	$ 22,466,841		$ 19,648,504
Ratio of loan type to all loans	100.00%		100.00%
Allowance for credit losses on loans	$ 203,824	$ 201,577	$ 177,207	$ 178,069	$ 177,420	$ 172,808
Loans in process	1,094,956		1,895,940
Net deferred fees, costs and discounts	294,142		98,807
Total loan contra accounts	1,592,922		2,171,954
Net loans	20,873,919		17,476,550
Commercial loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 13,235,910		$ 12,094,294
Ratio of loan type to all loans	58.90%		61.60%
Allowance for credit losses on loans	$ 153,195	149,413	$ 137,194	138,185	137,665	133,476
Commercial loans | Multi-family
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 4,616,359		$ 2,907,086
Ratio of loan type to all loans	20.50%		14.80%
Allowance for credit losses on loans	$ 25,199	21,979	$ 13,155	12,976	13,038	12,013
Commercial loans | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 3,781,247		$ 3,344,959
Ratio of loan type to all loans	16.80%		17.00%
Allowance for credit losses on loans	$ 39,756	32,991	$ 28,842	28,132	27,803	25,814
Commercial loans | Commercial & industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 2,394,978		$ 2,321,717
Ratio of loan type to all loans	10.70%		11.80%
Allowance for credit losses on loans	$ 57,833	59,261	$ 58,773	61,915	63,301	57,210
Commercial loans | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 2,247,530		$ 3,318,994
Ratio of loan type to all loans	10.00%		16.90%
Allowance for credit losses on loans	$ 22,707	27,317	$ 29,408	27,916	26,027	26,161
Commercial loans | Land - acquisition & development
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 195,796		$ 201,538
Ratio of loan type to all loans	0.90%		1.00%
Allowance for credit losses on loans	$ 7,700	7,865	$ 7,016	7,246	7,496	12,278
Consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 9,230,931		$ 7,554,210
Ratio of loan type to all loans	41.10%		38.40%
Allowance for credit losses on loans	$ 50,629	52,164	$ 40,013	39,884	39,755	39,332
Consumer loans | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 414,483		$ 672,643
Ratio of loan type to all loans	1.90%		3.40%
Allowance for credit losses on loans	$ 1,494	1,918	$ 2,781	3,221	3,456	3,410
Consumer loans | Single-family residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 8,364,415		$ 6,451,270
Ratio of loan type to all loans	37.20%		32.80%
Allowance for credit losses on loans	$ 40,316	41,054	$ 28,029	27,083	26,916	25,518
Consumer loans | Land - consumer lot loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 112,317		$ 125,723
Ratio of loan type to all loans	0.50%		0.60%
Allowance for credit losses on loans	$ 2,648	3,214	$ 3,512	3,770	3,945	5,047
Consumer loans | HELOC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 255,271		$ 234,410
Ratio of loan type to all loans	1.10%		1.20%
Allowance for credit losses on loans	$ 2,924	2,974	$ 2,859	2,743	2,662	2,482
Consumer loans | Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 84,445		$ 70,164
Ratio of loan type to all loans	0.40%		0.40%
Allowance for credit losses on loans	$ 3,247	$ 3,004	$ 2,832	$ 3,067	$ 2,776	$ 2,875