Allowance for Losses on Loans - Allowance for Credit Losses (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Allowance	$ 121,922	$ 109,919	$ 113,494	$ 106,829
Charge-offs	(411)	(1,175)	(1,908)	(5,234)
Recoveries	1,663	4,072	13,672	12,722
Provision & Transfers	(945)	(1,800)	(3,029)	(3,301)
Ending Allowance	122,229	111,016	122,229	111,016
Single-family residential
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Allowance	37,164	41,828	37,796	47,347
Charge-offs	(267)	(634)	(763)	(2,800)
Recoveries	81	162	455	2,739
Provision & Transfers	1,133	(675)	623	(6,605)
Ending Allowance	38,111	40,681	38,111	40,681
Construction
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Allowance	25,061	15,726	19,838	6,680
Charge-offs	0	0	0	0
Recoveries	0	207	0	357
Provision & Transfers	(3,195)	1,729	2,028	10,625
Ending Allowance	21,866	17,662	21,866	17,662
Construction - custom
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Allowance	1,176	1,022	1,080	990
Charge-offs	0	0	(3)	(60)
Recoveries	0	60	0	60
Provision & Transfers	714	(54)	813	38
Ending Allowance	1,890	1,028	1,890	1,028
Land - acquisition & development
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Allowance	6,669	7,252	6,023	5,781
Charge-offs	0	(31)	(63)	(31)
Recoveries	863	2,741	9,092	6,148
Provision & Transfers	(315)	(3,240)	(7,835)	(5,176)
Ending Allowance	7,217	6,722	7,217	6,722
Land - consumer lot loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Allowance	2,513	2,466	2,535	2,946
Charge-offs	0	(26)	(17)	(701)
Recoveries	118	5	368	5
Provision & Transfers	(83)	59	(338)	254
Ending Allowance	2,548	2,504	2,548	2,504
Multi-family
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Allowance	7,929	6,784	6,925	5,304
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision & Transfers	(17)	137	987	1,617
Ending Allowance	7,912	6,921	7,912	6,921
Commercial real estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Allowance	10,772	7,783	8,588	8,960
Charge-offs	0	0	(11)	(32)
Recoveries	164	454	1,684	1,569
Provision & Transfers	411	(94)	1,086	(2,354)
Ending Allowance	11,347	8,143	11,347	8,143
Commercial & industrial
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Allowance	28,365	23,824	28,008	24,980
Charge-offs	0	(150)	(163)	(729)
Recoveries	154	6	1,096	597
Provision & Transfers	653	716	231	(452)
Ending Allowance	29,172	24,396	29,172	24,396
HELOC
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Allowance	826	828	813	902
Charge-offs	0	(27)	(90)	(54)
Recoveries	1	0	2	21
Provision & Transfers	50	55	152	(13)
Ending Allowance	877	856	877	856
Consumer
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Allowance	1,447	2,406	1,888	2,939
Charge-offs	(144)	(307)	(798)	(827)
Recoveries	282	437	975	1,226
Provision & Transfers	(296)	(433)	(776)	(1,235)
Ending Allowance	$ 1,289	$ 2,103	$ 1,289	$ 2,103