Allowance for Losses on Loans - Allowance for Credit Losses (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2017	Mar. 31, 2016
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Allowance	$ 118,456	$ 107,901	$ 113,494	$ 106,829
Charge-offs	(1,093)	(1,938)	(1,497)	(4,059)
Recoveries	6,309	5,456	12,009	8,649
Provision & Transfers	(1,750)	(1,500)	(2,084)	(1,500)
Ending Allowance	121,922	109,919	121,922	109,919
Single-family residential
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Allowance	38,206	47,756	37,796	47,347
Charge-offs	(381)	(1,026)	(496)	(2,165)
Recoveries	223	111	374	2,577
Provision & Transfers	(884)	(5,013)	(510)	(5,931)
Ending Allowance	37,164	41,828	37,164	41,828
Construction
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Allowance	21,934	7,014	19,838	6,680
Charge-offs	0	0	0	0
Recoveries	0	(5)	0	150
Provision & Transfers	3,127	8,717	5,223	8,896
Ending Allowance	25,061	15,726	25,061	15,726
Construction - custom
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Allowance	1,110	1,062	1,080	990
Charge-offs	(3)	0	(3)	(60)
Recoveries	0	0	0	0
Provision & Transfers	69	(40)	99	92
Ending Allowance	1,176	1,022	1,176	1,022
Land - acquisition & development
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Allowance	6,665	6,778	6,023	5,781
Charge-offs	(43)	0	(63)	0
Recoveries	4,211	3,371	8,229	3,406
Provision & Transfers	(4,164)	(2,897)	(7,520)	(1,935)
Ending Allowance	6,669	7,252	6,669	7,252
Land - consumer lot loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Allowance	2,501	3,001	2,535	2,946
Charge-offs	0	(268)	(17)	(676)
Recoveries	180	0	250	0
Provision & Transfers	(168)	(267)	(255)	196
Ending Allowance	2,513	2,466	2,513	2,466
Multi-family
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Allowance	7,629	5,047	6,925	5,304
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision & Transfers	300	1,737	1,004	1,480
Ending Allowance	7,929	6,784	7,929	6,784
Commercial real estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Allowance	10,168	10,344	8,588	8,960
Charge-offs	0	(9)	(11)	(32)
Recoveries	1,164	992	1,520	1,115
Provision & Transfers	(560)	(3,544)	675	(2,260)
Ending Allowance	10,772	7,783	10,772	7,783
Commercial & industrial
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Allowance	27,736	24,096	28,008	24,980
Charge-offs	(105)	(331)	(163)	(579)
Recoveries	217	590	942	591
Provision & Transfers	517	(531)	(422)	(1,168)
Ending Allowance	28,365	23,824	28,365	23,824
HELOC
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Allowance	832	820	813	902
Charge-offs	(53)	(26)	(90)	(27)
Recoveries	0	0	1	21
Provision & Transfers	47	34	102	(68)
Ending Allowance	826	828	826	828
Consumer
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Allowance	1,675	1,983	1,888	2,939
Charge-offs	(508)	(278)	(654)	(520)
Recoveries	314	397	693	789
Provision & Transfers	(34)	304	(480)	(802)
Ending Allowance	$ 1,447	$ 2,406	$ 1,447	$ 2,406

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=66008989&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=66008989&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

No definition available.

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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=66008989&loc=SL6953423-111524

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- Definition

Financing Receivable, Allowance for Credit Losses, Provision and Transfers

+ References

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