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, 2020	Dec. 31, 2019
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	$ 166,955	$ 149,284
Charge-offs	(152)	(618)
Recoveries	1,886	3,197
Provision & Transfers	1,500	(4,350)
Ending Allowance	170,189	147,513
Commercial loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	109,923	89,335
Charge-offs	(2)	(159)
Recoveries	874	2,524
Provision & Transfers	8,412	(2,119)
Ending Allowance	119,207	89,581
Commercial loans \| Multi-family
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	13,853	10,404
Charge-offs	0	0
Recoveries	0	498
Provision & Transfers	510	(396)
Ending Allowance	14,363	10,506
Commercial loans \| Commercial real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	22,516	13,024
Charge-offs	0	(98)
Recoveries	789	368
Provision & Transfers	191	(227)
Ending Allowance	23,496	13,067
Commercial loans \| Commercial and Industrial [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	38,665	32,235
Charge-offs	(2)	(50)
Recoveries	50	144
Provision & Transfers	5,604	1,347
Ending Allowance	44,317	33,676
Commercial loans \| Construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	24,156	22,768
Charge-offs	0	0
Recoveries	0	54
Provision & Transfers	2,209	(903)
Ending Allowance	26,365	21,919
Commercial loans \| Land - acquisition & development
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	10,733	10,904
Charge-offs	0	(11)
Recoveries	35	1,460
Provision & Transfers	(102)	(1,940)
Ending Allowance	10,666	10,413
Consumer loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	57,032	59,949
Charge-offs	(150)	(459)
Recoveries	1,012	673
Provision & Transfers	(6,912)	(2,231)
Ending Allowance	50,982	57,932
Consumer loans \| Construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	3,555	2,880
Charge-offs	0	0
Recoveries	0	0
Provision & Transfers	39	50
Ending Allowance	3,594	2,930
Consumer loans \| Single-family residential
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	45,186	47,771
Charge-offs	0	(15)
Recoveries	779	261
Provision & Transfers	(7,352)	(1,661)
Ending Allowance	38,613	46,356
Consumer loans \| Land - consumer lot loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	2,729	2,635
Charge-offs	0	(70)
Recoveries	7	10
Provision & Transfers	222	(8)
Ending Allowance	2,958	2,567
Consumer loans \| HELOC
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	2,571	2,048
Charge-offs	0	0
Recoveries	0	93
Provision & Transfers	(209)	(107)
Ending Allowance	2,362	2,034
Consumer loans \| Consumer
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	2,991	4,615
Charge-offs	(150)	(374)
Recoveries	226	309
Provision & Transfers	388	(505)
Ending Allowance	$ 3,455	$ 4,045

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

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