Allowance for Losses on Loans - Allowance for Credit Losses (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Allowance	$ 131,165	$ 127,155	$ 129,257	$ 123,073
Charge-offs	(1,015)	(439)	(2,656)	(1,191)
Recoveries	2,186	1,810	5,235	5,644
Provision & Transfers	750	(950)	1,250	50
Ending Allowance	133,086	127,576	133,086	127,576
Single-family residential
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Allowance	31,484	35,928	33,033	36,892
Charge-offs	(150)	(290)	(175)	(750)
Recoveries	310	211	539	331
Provision & Transfers	(168)	(1,705)	(1,921)	(2,329)
Ending Allowance	31,476	34,144	31,476	34,144
Construction
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Allowance	31,463	25,214	31,317	24,556
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision & Transfers	1,933	2,175	2,079	2,833
Ending Allowance	33,396	27,389	33,396	27,389
Construction - custom
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Allowance	1,926	2,052	1,842	1,944
Charge-offs	0	0	0	(50)
Recoveries	0	0	0	0
Provision & Transfers	50	29	134	187
Ending Allowance	1,976	2,081	1,976	2,081
Land - acquisition & development
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Allowance	9,156	7,355	7,978	6,829
Charge-offs	0	0	0	0
Recoveries	1,300	1,207	3,082	4,579
Provision & Transfers	(722)	(940)	(1,326)	(3,786)
Ending Allowance	9,734	7,622	9,734	7,622
Land - consumer lot loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Allowance	2,144	2,906	2,164	2,649
Charge-offs	(48)	(18)	(120)	(66)
Recoveries	0	0	265	0
Provision & Transfers	(20)	(35)	(233)	270
Ending Allowance	2,076	2,853	2,076	2,853
Multi-family
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Allowance	7,884	7,904	8,329	7,862
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision & Transfers	(490)	78	(935)	120
Ending Allowance	7,394	7,982	7,394	7,982
Commercial real estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Allowance	12,711	11,625	11,852	11,818
Charge-offs	0	(36)	(339)	(36)
Recoveries	244	1	770	1
Provision & Transfers	(507)	(2)	165	(195)
Ending Allowance	12,448	11,588	12,448	11,588
Commercial & industrial
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Allowance	30,279	29,268	28,702	28,524
Charge-offs	(285)	0	(464)	(116)
Recoveries	24	115	58	170
Provision & Transfers	556	(53)	2,278	752
Ending Allowance	30,574	29,330	30,574	29,330
HELOC
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Allowance	1,064	808	781	855
Charge-offs	(200)	(1)	(1,086)	(1)
Recoveries	43	0	44	1
Provision & Transfers	175	(5)	1,343	(53)
Ending Allowance	1,082	802	1,082	802
Consumer
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Allowance	3,054	4,095	3,259	1,144
Charge-offs	(332)	(94)	(472)	(172)
Recoveries	265	276	477	562
Provision & Transfers	(57)	(492)	(334)	2,251
Ending Allowance	$ 2,930	$ 3,785	$ 2,930	$ 3,785

A valuation allowance for financing receivables that are expected to be uncollectible.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

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- Definition

Amount of recovery of financing receivables doubtful of collection that were previously charged off.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(4)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

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- Definition

Amount of direct write-downs of financing receivables charged against the allowance.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

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- Definition

Financing Receivable, Allowance for Credit Losses, Provision and Transfers

+ References

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