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, 2023	Mar. 31, 2022	Mar. 31, 2023	Mar. 31, 2022
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	$ 176,797	$ 171,411	$ 172,808	$ 171,300
Charge-offs	(6,126)	(912)	(6,354)	(1,589)
Recoveries	249	1,385	966	4,173
Provision & Transfers	3,500	(500)	6,000	0
Ending Allowance	177,420	171,384	177,420	171,384
Provisions & Transfers	6,500	(500)	10,000	(2,500)
Unfunded commitments
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Provisions & Transfers	(3,000)		(4,000)	2,500
Commercial loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	138,282	127,837	133,476	125,375
Charge-offs	(6,054)	(873)	(6,136)	(1,447)
Recoveries	57	954	109	3,080
Ending Allowance	137,665	129,440	137,665	129,440
Provisions & Transfers	5,380	1,522	10,216	2,432
Commercial loans | Multi-family
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	12,324	15,993	12,013	16,949
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Ending Allowance	13,038	14,463	13,038	14,463
Provisions & Transfers	714	(1,530)	1,025	(2,486)
Commercial loans | Commercial real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	27,380	25,722	25,814	23,437
Charge-offs	0	0	0	(529)
Recoveries	1	753	5	797
Ending Allowance	27,803	24,682	27,803	24,682
Provisions & Transfers	422	(1,793)	1,984	977
Commercial loans | Commercial & industrial
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	63,873	47,231	57,210	45,957
Charge-offs	(6,054)	(873)	(6,136)	(916)
Recoveries	42	2	74	64
Ending Allowance	63,301	53,903	63,301	53,903
Provisions & Transfers	5,440	7,543	12,153	8,798
Commercial loans | Construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	26,133	24,766	26,161	25,585
Charge-offs	0	0	0	0
Recoveries	0	179	0	2,179
Ending Allowance	26,027	22,707	26,027	22,707
Provisions & Transfers	(106)	(2,238)	(134)	(5,057)
Commercial loans | Land - acquisition & development
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	8,572	14,125	12,278	13,447
Charge-offs	0	0	0	(2)
Recoveries	14	20	30	40
Ending Allowance	7,496	13,685	7,496	13,685
Provisions & Transfers	(1,090)	(460)	(4,812)	200
Consumer loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	38,515	43,574	39,332	45,925
Charge-offs	(72)	(39)	(218)	(142)
Recoveries	192	431	857	1,093
Ending Allowance	39,755	41,944	39,755	41,944
Provisions & Transfers	1,120	(2,022)	(216)	(4,932)
Consumer loans | Construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	3,500	3,719	3,410	4,907
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Ending Allowance	3,456	3,129	3,456	3,129
Provisions & Transfers	(44)	(590)	46	(1,778)
Consumer loans | Single-family residential
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	25,475	30,106	25,518	30,978
Charge-offs	(34)	0	(34)	0
Recoveries	104	147	534	552
Ending Allowance	26,916	28,992	26,916	28,992
Provisions & Transfers	1,371	(1,261)	898	(2,538)
Consumer loans | Land - consumer lot loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	4,142	4,984	5,047	4,939
Charge-offs	0	0	0	(27)
Recoveries	5	40	5	45
Ending Allowance	3,945	5,105	3,945	5,105
Provisions & Transfers	(202)	81	(1,107)	148
Consumer loans | HELOC
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	2,588	2,365	2,482	2,390
Charge-offs	0	0	0	0
Recoveries	0	100	1	101
Ending Allowance	2,662	2,475	2,662	2,475
Provisions & Transfers	74	10	179	(16)
Consumer loans | Consumer
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	2,810	2,400	2,875	2,711
Charge-offs	(38)	(39)	(184)	(115)
Recoveries	83	144	317	395
Ending Allowance	2,776	2,243	2,776	2,243
Provisions & Transfers	$ (79)	$ (262)	$ (232)	$ (748)