Allowance for Loan Losses - Loans Based on Credit Quality Indicators (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	$ 17,653,757	$ 16,286,372
Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	$ 17,246,669	$ 16,045,127
Pass | Accounts Receivable | Credit Concentration Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Total grade as a % of total loans	97.70%	98.50%
Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	$ 101,922	$ 33,250
Special Mention | Accounts Receivable | Credit Concentration Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Total grade as a % of total loans	0.60%	0.20%
Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	$ 305,166	$ 207,995
Substandard | Accounts Receivable | Credit Concentration Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Total grade as a % of total loans	1.70%	1.30%
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	$ 10,507,610	$ 9,728,501
Commercial loans | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	10,119,666	9,508,927
Commercial loans | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	101,922	33,250
Commercial loans | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	286,022	186,324
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	2,886,594	2,626,479
Commercial loans | Multi-family | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	2,772,538	2,612,669
Commercial loans | Multi-family | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	90,428	0
Commercial loans | Multi-family | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	23,628	13,810
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	3,310,101	3,111,112
Commercial loans | Commercial real estate | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	3,207,596	3,054,241
Commercial loans | Commercial real estate | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	4,847	1,594
Commercial loans | Commercial real estate | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	97,658	55,277
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	2,315,318	2,343,403
Commercial loans | Commercial & industrial | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	2,164,876	2,196,767
Commercial loans | Commercial & industrial | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	6,647	31,656
Commercial loans | Commercial & industrial | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	143,795	114,980
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	1,838,936	1,423,891
Commercial loans | Construction | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	1,818,266	1,421,634
Commercial loans | Construction | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	0	0
Commercial loans | Construction | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	20,670	2,257
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	156,661	223,616
Commercial loans | Land - acquisition & development | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	156,390	223,616
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	271	0
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	7,146,147	6,557,871
Consumer loans | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	7,127,003	6,536,200
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	19,144	21,671
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	324,451	397,343
Consumer loans | Construction | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	324,363	396,908
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	88	435
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	6,388,990	5,726,979
Consumer loans | Single-family residential | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	6,370,936	5,706,199
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	18,054	20,780
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	124,842	151,945
Consumer loans | Land - consumer lot loans | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	124,588	151,723
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	254	222
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	237,754	206,033
Consumer loans | HELOC | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	237,018	205,800
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	736	233
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	70,110	75,571
Consumer loans | Consumer | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	70,098	75,570
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	12	1
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