Allowance for Losses on Loans - Activity in Allowance for Loan Losses by Loan Portfolio Segment and Class (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Mar. 31, 2022	Mar. 31, 2021	Mar. 31, 2022	Mar. 31, 2021
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	$ 171,411	$ 170,189	$ 171,300	$ 166,955
Charge-offs	(912)	(208)	(1,589)	(360)
Recoveries	1,385	2,672	4,173	4,558
Provision & Transfers	(500)	0	(2,500)	1,500
Ending Allowance	171,384	172,653	171,384	172,653
Unfunded commitments
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Provision & Transfers	0	0	2,500	1,500
Commercial loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	127,837	119,207	125,375	109,923
Charge-offs	(873)	(18)	(1,447)	(20)
Recoveries	954	1,878	3,080	2,752
Provision & Transfers	1,522	4,377	2,432	12,789
Ending Allowance	129,440	125,444	129,440	125,444
Commercial loans \| Multi-family
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	15,993	14,363	16,949	13,853
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision & Transfers	(1,530)	3,413	(2,486)	3,923
Ending Allowance	14,463	17,776	14,463	17,776
Commercial loans \| Commercial real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	25,722	23,496	23,437	22,516
Charge-offs	0	0	(529)	0
Recoveries	753	1,457	797	2,246
Provision & Transfers	(1,793)	1,869	977	2,060
Ending Allowance	24,682	26,822	24,682	26,822
Commercial loans \| Commercial & industrial
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	47,231	44,317	45,957	38,665
Charge-offs	(873)	(18)	(916)	(20)
Recoveries	2	11	64	61
Provision & Transfers	7,543	3,484	8,798	9,088
Ending Allowance	53,903	47,794	53,903	47,794
Commercial loans \| Construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	24,766	26,365	25,585	24,156
Charge-offs	0	0	0	0
Recoveries	179	0	2,179	0
Provision & Transfers	(2,238)	(3,816)	(5,057)	(1,607)
Ending Allowance	22,707	22,549	22,707	22,549
Commercial loans \| Land - acquisition & development
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	14,125	10,666	13,447	10,733
Charge-offs	0	0	(2)	0
Recoveries	20	410	40	445
Provision & Transfers	(460)	(573)	200	(675)
Ending Allowance	13,685	10,503	13,685	10,503
Consumer loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	43,574	50,982	45,925	57,032
Charge-offs	(39)	(190)	(142)	(340)
Recoveries	431	794	1,093	1,806
Provision & Transfers	(2,022)	(4,377)	(4,932)	(11,289)
Ending Allowance	41,944	47,209	41,944	47,209
Consumer loans \| Construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	3,719	3,594	4,907	3,555
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision & Transfers	(590)	(346)	(1,778)	(307)
Ending Allowance	3,129	3,248	3,129	3,248
Consumer loans \| Single-family residential
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	30,106	38,613	30,978	45,186
Charge-offs	0	(106)	0	(106)
Recoveries	147	497	552	1,276
Provision & Transfers	(1,261)	(3,896)	(2,538)	(11,248)
Ending Allowance	28,992	35,108	28,992	35,108
Consumer loans \| Land - consumer lot loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	4,984	2,958	4,939	2,729
Charge-offs	0	0	(27)	0
Recoveries	40	7	45	14
Provision & Transfers	81	327	148	549
Ending Allowance	5,105	3,292	5,105	3,292
Consumer loans \| HELOC
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	2,365	2,362	2,390	2,571
Charge-offs	0	0	0	0
Recoveries	100	0	101	0
Provision & Transfers	10	(136)	(16)	(345)
Ending Allowance	2,475	2,226	2,475	2,226
Consumer loans \| Consumer
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	2,400	3,455	2,711	2,991
Charge-offs	(39)	(84)	(115)	(234)
Recoveries	144	290	395	516
Provision & Transfers	(262)	(326)	(748)	62
Ending Allowance	$ 2,243	$ 3,335	$ 2,243	$ 3,335

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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