Allowance for Losses on Loans - Allowance for Credit Losses (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Allowance	$ 127,155	$ 118,456	$ 123,073	$ 113,494
Charge-offs	(439)	(1,093)	(1,191)	(1,497)
Recoveries	1,810	6,309	5,644	12,009
Provision & Transfers	(950)	(1,750)	50	(2,084)
Ending Allowance	127,576	121,922	127,576	121,922
Single-family residential
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Allowance	35,928	38,206	36,892	37,796
Charge-offs	(290)	(381)	(750)	(496)
Recoveries	211	223	331	374
Provision & Transfers	(1,705)	(884)	(2,329)	(510)
Ending Allowance	34,144	37,164	34,144	37,164
Construction
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Allowance	25,214	21,934	24,556	19,838
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision & Transfers	2,175	3,127	2,833	5,223
Ending Allowance	27,389	25,061	27,389	25,061
Construction - custom
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Allowance	2,052	1,110	1,944	1,080
Charge-offs	0	(3)	(50)	(3)
Recoveries	0	0	0	0
Provision & Transfers	29	69	187	99
Ending Allowance	2,081	1,176	2,081	1,176
Land - acquisition & development
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Allowance	7,355	6,665	6,829	6,023
Charge-offs	0	(43)	0	(63)
Recoveries	1,207	4,211	4,579	8,229
Provision & Transfers	(940)	(4,164)	(3,786)	(7,520)
Ending Allowance	7,622	6,669	7,622	6,669
Land - consumer lot loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Allowance	2,906	2,501	2,649	2,535
Charge-offs	(18)	0	(66)	(17)
Recoveries	0	180	0	250
Provision & Transfers	(35)	(168)	270	(255)
Ending Allowance	2,853	2,513	2,853	2,513
Multi-family
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Allowance	7,904	7,629	7,862	6,925
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision & Transfers	78	300	120	1,004
Ending Allowance	7,982	7,929	7,982	7,929
Commercial real estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Allowance	11,625	10,168	11,818	8,588
Charge-offs	(36)	0	(36)	(11)
Recoveries	1	1,164	1	1,520
Provision & Transfers	(2)	(560)	(195)	675
Ending Allowance	11,588	10,772	11,588	10,772
Commercial & industrial
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Allowance	29,268	27,736	28,524	28,008
Charge-offs	0	(105)	(116)	(163)
Recoveries	115	217	170	942
Provision & Transfers	(53)	517	752	(422)
Ending Allowance	29,330	28,365	29,330	28,365
HELOC
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Allowance	808	832	855	813
Charge-offs	(1)	(53)	(1)	(90)
Recoveries	0	0	1	1
Provision & Transfers	(5)	47	(53)	102
Ending Allowance	802	826	802	826
Consumer
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Allowance	4,095	1,675	1,144	1,888
Charge-offs	(94)	(508)	(172)	(654)
Recoveries	276	314	562	693
Provision & Transfers	(492)	(34)	2,251	(480)
Ending Allowance	$ 3,785	$ 1,447	$ 3,785	$ 1,447

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-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://www.xbrl.org/2003/role/presentationRef
-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://www.xbrl.org/2003/role/presentationRef
-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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