Allowance for Losses on Loans - Allowance for Credit Losses (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance (After ASC 326 Adoption)	$ 157,101	$ 133,086	$ 131,534	$ 129,257
Charge-offs	(3,327)	(4,752)	(4,395)	(7,408)
Recoveries	1,625	5,688	7,060	10,923
Provision & Transfers	9,950	0	13,400	1,250
Ending Allowance (After ASC 326 Adoption)	165,349	134,022	165,349	134,022
Commercial loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance (After ASC 326 Adoption)	98,704	93,546	93,470	88,178
Charge-offs	(3,034)	(4,099)	(3,335)	(4,903)
Recoveries	800	5,333	4,532	9,243
Provision & Transfers	9,938	23	15,876	2,285
Ending Allowance (After ASC 326 Adoption)	106,408	94,803	106,408	94,803
Commercial loans \| Multi-family
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance (After ASC 326 Adoption)	11,742	7,394	7,391	8,329
Charge-offs	0	0	0	0
Recoveries	0	0	498	0
Provision & Transfers	346	(111)	1,186	(1,046)
Ending Allowance (After ASC 326 Adoption)	12,088	7,283	12,088	7,283
Commercial loans \| Commercial real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance (After ASC 326 Adoption)	14,639	12,448	13,170	11,852
Charge-offs	0	0	(111)	(339)
Recoveries	193	90	1,581	860
Provision & Transfers	975	466	1,313	631
Ending Allowance (After ASC 326 Adoption)	15,807	13,004	15,807	13,004
Commercial loans \| Commercial & industrial
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance (After ASC 326 Adoption)	38,576	30,574	31,450	28,702
Charge-offs	(3,034)	(4,034)	(3,213)	(4,499)
Recoveries	174	3,218	375	3,276
Provision & Transfers	6,463	1,313	12,782	3,592
Ending Allowance (After ASC 326 Adoption)	42,179	31,071	42,179	31,071
Commercial loans \| Construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance (After ASC 326 Adoption)	23,348	33,396	32,304	31,317
Charge-offs	0	0	0	0
Recoveries	0	0	59	0
Provision & Transfers	2,345	661	2,866	2,740
Ending Allowance (After ASC 326 Adoption)	25,693	34,057	25,693	34,057
Commercial loans \| Land - acquisition & development
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance (After ASC 326 Adoption)	10,399	9,734	9,155	7,978
Charge-offs	0	(65)	(11)	(65)
Recoveries	433	2,025	2,019	5,107
Provision & Transfers	(191)	(2,306)	(2,271)	(3,632)
Ending Allowance (After ASC 326 Adoption)	10,641	9,388	10,641	9,388
Consumer loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance (After ASC 326 Adoption)	58,397	39,540	38,064	41,079
Charge-offs	(293)	(653)	(1,060)	(2,505)
Recoveries	825	355	2,528	1,680
Provision & Transfers	12	(23)	(2,476)	(1,035)
Ending Allowance (After ASC 326 Adoption)	58,941	39,219	58,941	39,219
Consumer loans \| Construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance (After ASC 326 Adoption)	3,175	1,976	1,369	1,842
Charge-offs	0	(339)	0	(339)
Recoveries	0	0	0	0
Provision & Transfers	161	247	456	381
Ending Allowance (After ASC 326 Adoption)	3,336	1,884	3,336	1,884
Consumer loans \| Single-family residential
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance (After ASC 326 Adoption)	46,817	31,476	30,988	33,033
Charge-offs	(60)	(65)	(75)	(238)
Recoveries	437	47	891	586
Provision & Transfers	(45)	56	(1,438)	(1,867)
Ending Allowance (After ASC 326 Adoption)	47,149	31,514	47,149	31,514
Consumer loans \| Land - consumer lot loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance (After ASC 326 Adoption)	2,578	2,076	2,143	2,164
Charge-offs	0	(215)	(147)	(336)
Recoveries	17	0	503	265
Provision & Transfers	76	171	(320)	(61)
Ending Allowance (After ASC 326 Adoption)	2,671	2,032	2,671	2,032
Consumer loans \| HELOC
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance (After ASC 326 Adoption)	2,246	1,082	1,103	781
Charge-offs	0	0	0	(1,086)
Recoveries	1	1	95	45
Provision & Transfers	341	4	445	1,347
Ending Allowance (After ASC 326 Adoption)	2,588	1,087	2,588	1,087
Consumer loans \| Consumer
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Allowance (After ASC 326 Adoption)	3,581	2,930	2,461	3,259
Charge-offs	(233)	(34)	(838)	(506)
Recoveries	370	307	1,039	784
Provision & Transfers	(521)	(501)	(1,619)	(835)
Ending Allowance (After ASC 326 Adoption)	$ 3,197	$ 2,702	$ 3,197	$ 2,702

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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