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, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	$ 170,189	$ 147,513	$ 166,955	$ 149,284
Charge-offs	(208)	(449)	(360)	(1,067)
Recoveries	2,672	2,237	4,558	5,434
Provision & Transfers	0	7,800	1,500	3,450
Ending Allowance	172,653	157,101	172,653	157,101
Unfunded commitments
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Provision & Transfers		400	1,500	1,000
Commercial loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	119,207	89,581	109,923	89,335
Charge-offs	(18)	(142)	(20)	(301)
Recoveries	1,878	1,208	2,752	3,732
Provision & Transfers	4,377	8,057	12,789	5,938
Ending Allowance	125,444	98,704	125,444	98,704
Commercial loans \| Multi-family
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	14,363	10,506	13,853	10,404
Charge-offs	0	0	0	0
Recoveries	0	0	0	498
Provision & Transfers	3,413	1,236	3,923	840
Ending Allowance	17,776	11,742	17,776	11,742
Commercial loans \| Commercial real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	23,496	13,067	22,516	13,024
Charge-offs	0	(13)	0	(111)
Recoveries	1,457	1,020	2,246	1,388
Provision & Transfers	1,869	565	2,060	338
Ending Allowance	26,822	14,639	26,822	14,639
Commercial loans \| Commercial & industrial
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	44,317	33,676	38,665	32,235
Charge-offs	(18)	(129)	(20)	(179)
Recoveries	11	57	61	201
Provision & Transfers	3,484	4,972	9,088	6,319
Ending Allowance	47,794	38,576	47,794	38,576
Commercial loans \| Construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	26,365	21,919	24,156	22,768
Charge-offs	0	0	0	0
Recoveries	0	5	0	59
Provision & Transfers	(3,816)	1,424	(1,607)	521
Ending Allowance	22,549	23,348	22,549	23,348
Commercial loans \| Land - acquisition & development
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	10,666	10,413	10,733	10,904
Charge-offs	0	0	0	(11)
Recoveries	410	126	445	1,586
Provision & Transfers	(573)	(140)	(675)	(2,080)
Ending Allowance	10,503	10,399	10,503	10,399
Consumer loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	50,982	57,932	57,032	59,949
Charge-offs	(190)	(307)	(340)	(766)
Recoveries	794	1,029	1,806	1,702
Provision & Transfers	(4,377)	(257)	(11,289)	(2,488)
Ending Allowance	47,209	58,397	47,209	58,397
Consumer loans \| Construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	3,594	2,930	3,555	2,880
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision & Transfers	(346)	245	(307)	295
Ending Allowance	3,248	3,175	3,248	3,175
Consumer loans \| Single-family residential
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	38,613	46,356	45,186	47,771
Charge-offs	(106)	0	(106)	(15)
Recoveries	497	192	1,276	453
Provision & Transfers	(3,896)	268	(11,248)	(1,393)
Ending Allowance	35,108	46,816	35,108	46,816
Consumer loans \| Land - consumer lot loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	2,958	2,567	2,729	2,635
Charge-offs	0	(77)	0	(147)
Recoveries	7	476	14	486
Provision & Transfers	327	(388)	549	(396)
Ending Allowance	3,292	2,578	3,292	2,578
Consumer loans \| HELOC
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	2,362	2,034	2,571	2,048
Charge-offs	0	0	0	0
Recoveries	0	1	0	94
Provision & Transfers	(136)	211	(345)	104
Ending Allowance	2,226	2,246	2,226	2,246
Consumer loans \| Consumer
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance	3,455	4,045	2,991	4,615
Charge-offs	(84)	(230)	(234)	(604)
Recoveries	290	360	516	669
Provision & Transfers	(326)	(593)	62	(1,098)
Ending Allowance	$ 3,335	$ 3,582	$ 3,335	$ 3,582

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

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