Allowance for Losses on Loans - Activity in Allowance for Loan Losses by Loan Portfolio Segment and Class (Details) - USD ($) $ in Thousands	3 Months Ended
Dec. 31, 2021	Dec. 31, 2020
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	$ 171,300	$ 166,955
Charge-offs	(677)	(152)
Recoveries	2,788	1,886
Provision & Transfers	(2,000)	1,500
Ending Allowance	171,411	170,189
Unfunded commitments
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Provision & Transfers	2,500	1,500
Commercial loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	125,375	109,923
Charge-offs	(574)	(2)
Recoveries	2,126	874
Provision & Transfers	910	8,412
Ending Allowance	127,837	119,207
Commercial loans \| Multi-family
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	16,949	13,853
Charge-offs	0	0
Recoveries	0	0
Provision & Transfers	(956)	510
Ending Allowance	15,993	14,363
Commercial loans \| Commercial real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	23,437	22,516
Charge-offs	(529)	0
Recoveries	44	789
Provision & Transfers	2,770	191
Ending Allowance	25,722	23,496
Commercial loans \| Commercial & industrial
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	45,957	38,665
Charge-offs	(43)	(2)
Recoveries	62	50
Provision & Transfers	1,255	5,604
Ending Allowance	47,231	44,317
Commercial loans \| Construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	25,585	24,156
Charge-offs	0	0
Recoveries	2,000	0
Provision & Transfers	(2,819)	2,209
Ending Allowance	24,766	26,365
Commercial loans \| Land - acquisition & development
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	13,447	10,733
Charge-offs	(2)	0
Recoveries	20	35
Provision & Transfers	660	(102)
Ending Allowance	14,125	10,666
Consumer loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	45,925	57,032
Charge-offs	(103)	(150)
Recoveries	662	1,012
Provision & Transfers	(2,910)	(6,912)
Ending Allowance	43,574	50,982
Consumer loans \| Construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	4,907	3,555
Charge-offs	0	0
Recoveries	0	0
Provision & Transfers	(1,188)	39
Ending Allowance	3,719	3,594
Consumer loans \| Single-family residential
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	30,978	45,186
Charge-offs	0	0
Recoveries	405	779
Provision & Transfers	(1,277)	(7,352)
Ending Allowance	30,106	38,613
Consumer loans \| Land - consumer lot loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	4,939	2,729
Charge-offs	(27)	0
Recoveries	5	7
Provision & Transfers	67	222
Ending Allowance	4,984	2,958
Consumer loans \| HELOC
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	2,390	2,571
Charge-offs	0	0
Recoveries	1	0
Provision & Transfers	(26)	(209)
Ending Allowance	2,365	2,362
Consumer loans \| Consumer
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	2,711	2,991
Charge-offs	(76)	(150)
Recoveries	251	226
Provision & Transfers	(486)	388
Ending Allowance	$ 2,400	$ 3,455

Amount of allowance for credit loss on financing receivable. Excludes allowance for financing receivable covered under loss sharing agreement.

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Amount of increase in allowance for credit loss on financing receivable from recovery.

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Amount of writeoff of financing receivable, charged against allowance for credit loss.

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