Allowance for Losses on Loans - Impaired Loans Based on Loan Types (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Jun. 30, 2018	Sep. 30, 2017
Financing Receivable, Impaired [Line Items]
Recorded Investment	$ 267,527	$ 251,274
Unpaid Principal Balance	281,484	289,340
Related Allowance	2,874	4,156
Average Recorded Investment (Year-To-Date)	262,946	274,530
Amount of related allowance included in specific reserves	82	126
Amount of related allowance included in general reserves	2,792	4,030
Impaired loans with no related allowance recorded
Financing Receivable, Impaired [Line Items]
Recorded Investment	95,924	43,897
Unpaid Principal Balance	102,290	75,267
Related Allowance	0	0
Average Recorded Investment (Year-To-Date)	70,777	40,392
Impaired loans with an allowance recorded
Financing Receivable, Impaired [Line Items]
Recorded Investment	171,603	207,377
Unpaid Principal Balance	179,194	214,073
Related Allowance	2,874	4,156
Average Recorded Investment (Year-To-Date)	192,169	234,138
Single-family residential
Financing Receivable, Impaired [Line Items]
Recorded Investment	175,470	203,266
Unpaid Principal Balance	179,954	210,047
Related Allowance	2,792	4,030
Average Recorded Investment (Year-To-Date)	189,155	224,094
Single-family residential | Impaired loans with no related allowance recorded
Financing Receivable, Impaired [Line Items]
Recorded Investment	22,114	21,325
Unpaid Principal Balance	23,310	23,880
Related Allowance	0	0
Average Recorded Investment (Year-To-Date)	21,944	19,371
Single-family residential | Impaired loans with an allowance recorded
Financing Receivable, Impaired [Line Items]
Recorded Investment	153,356	181,941
Unpaid Principal Balance	156,644	186,167
Related Allowance	2,792	4,030
Average Recorded Investment (Year-To-Date)	167,211	204,723
Construction
Financing Receivable, Impaired [Line Items]
Recorded Investment	4,843
Unpaid Principal Balance	4,933
Related Allowance	0
Average Recorded Investment (Year-To-Date)	2,928
Construction | Impaired loans with no related allowance recorded
Financing Receivable, Impaired [Line Items]
Recorded Investment	4,843
Unpaid Principal Balance	4,933
Related Allowance	0
Average Recorded Investment (Year-To-Date)	2,928
Construction - custom
Financing Receivable, Impaired [Line Items]
Recorded Investment	0	148
Unpaid Principal Balance	0	165
Related Allowance	0	0
Average Recorded Investment (Year-To-Date)	37	231
Construction - custom | Impaired loans with no related allowance recorded
Financing Receivable, Impaired [Line Items]
Recorded Investment	0	148
Unpaid Principal Balance	0	165
Related Allowance	0	0
Average Recorded Investment (Year-To-Date)	37	231
Land - acquisition & development
Financing Receivable, Impaired [Line Items]
Recorded Investment	3,073	420
Unpaid Principal Balance	3,120	8,298
Related Allowance	0	1
Average Recorded Investment (Year-To-Date)	1,764	752
Land - acquisition & development | Impaired loans with no related allowance recorded
Financing Receivable, Impaired [Line Items]
Recorded Investment	3,073	330
Unpaid Principal Balance	3,120	8,208
Related Allowance	0	0
Average Recorded Investment (Year-To-Date)	1,741	176
Land - acquisition & development | Impaired loans with an allowance recorded
Financing Receivable, Impaired [Line Items]
Recorded Investment	0	90
Unpaid Principal Balance	0	90
Related Allowance	0	1
Average Recorded Investment (Year-To-Date)	23	576
Land - consumer lot loans
Financing Receivable, Impaired [Line Items]
Recorded Investment	5,591	8,157
Unpaid Principal Balance	5,992	8,856
Related Allowance	0	0
Average Recorded Investment (Year-To-Date)	7,169	9,407
Land - consumer lot loans | Impaired loans with no related allowance recorded
Financing Receivable, Impaired [Line Items]
Recorded Investment	455	208
Unpaid Principal Balance	490	330
Related Allowance	0	0
Average Recorded Investment (Year-To-Date)	337	431
Land - consumer lot loans | Impaired loans with an allowance recorded
Financing Receivable, Impaired [Line Items]
Recorded Investment	5,136	7,949
Unpaid Principal Balance	5,502	8,526
Related Allowance	0	0
Average Recorded Investment (Year-To-Date)	6,832	8,976
Multi-family
Financing Receivable, Impaired [Line Items]
Recorded Investment	3,067	632
Unpaid Principal Balance	3,067	3,724
Related Allowance	5	5
Average Recorded Investment (Year-To-Date)	1,880	1,772
Multi-family | Impaired loans with no related allowance recorded
Financing Receivable, Impaired [Line Items]
Recorded Investment	2,606	139
Unpaid Principal Balance	2,606	3,231
Related Allowance	0	0
Average Recorded Investment (Year-To-Date)	1,403	748
Multi-family | Impaired loans with an allowance recorded
Financing Receivable, Impaired [Line Items]
Recorded Investment	461	493
Unpaid Principal Balance	461	493
Related Allowance	5	5
Average Recorded Investment (Year-To-Date)	477	1,024
Commercial real estate
Financing Receivable, Impaired [Line Items]
Recorded Investment	33,980	27,969
Unpaid Principal Balance	39,347	39,194
Related Allowance	77	120
Average Recorded Investment (Year-To-Date)	33,830	28,457
Commercial real estate | Impaired loans with no related allowance recorded
Financing Receivable, Impaired [Line Items]
Recorded Investment	26,818	12,890
Unpaid Principal Balance	31,372	22,487
Related Allowance	0	0
Average Recorded Investment (Year-To-Date)	22,338	11,466
Commercial real estate | Impaired loans with an allowance recorded
Financing Receivable, Impaired [Line Items]
Recorded Investment	7,162	15,079
Unpaid Principal Balance	7,975	16,707
Related Allowance	77	120
Average Recorded Investment (Year-To-Date)	11,492	16,991
Commercial & industrial
Financing Receivable, Impaired [Line Items]
Recorded Investment	39,351	8,279
Unpaid Principal Balance	42,700	14,321
Related Allowance	0	0
Average Recorded Investment (Year-To-Date)	24,054	7,722
Commercial & industrial | Impaired loans with no related allowance recorded
Financing Receivable, Impaired [Line Items]
Recorded Investment	35,427	8,279
Unpaid Principal Balance	35,736	14,321
Related Allowance	0	0
Average Recorded Investment (Year-To-Date)	19,507	7,425
Commercial & industrial | Impaired loans with an allowance recorded
Financing Receivable, Impaired [Line Items]
Recorded Investment	3,924	0
Unpaid Principal Balance	6,964	0
Related Allowance	0	0
Average Recorded Investment (Year-To-Date)	4,547	297
HELOC
Financing Receivable, Impaired [Line Items]
Recorded Investment	2,053	2,218
Unpaid Principal Balance	2,230	3,018
Related Allowance	0	0
Average Recorded Investment (Year-To-Date)	1,986	1,938
HELOC | Impaired loans with no related allowance recorded
Financing Receivable, Impaired [Line Items]
Recorded Investment	562	490
Unpaid Principal Balance	655	1,212
Related Allowance	0	0
Average Recorded Investment (Year-To-Date)	486	487
HELOC | Impaired loans with an allowance recorded
Financing Receivable, Impaired [Line Items]
Recorded Investment	1,491	1,728
Unpaid Principal Balance	1,575	1,806
Related Allowance	0	0
Average Recorded Investment (Year-To-Date)	1,500	1,451
Consumer
Financing Receivable, Impaired [Line Items]
Recorded Investment	99	185
Unpaid Principal Balance	141	1,717
Related Allowance	0	0
Average Recorded Investment (Year-To-Date)	143	157
Consumer | Impaired loans with no related allowance recorded
Financing Receivable, Impaired [Line Items]
Recorded Investment	26	88
Unpaid Principal Balance	68	1,433
Related Allowance	0	0
Average Recorded Investment (Year-To-Date)	56	57
Consumer | Impaired loans with an allowance recorded
Financing Receivable, Impaired [Line Items]
Recorded Investment	73	97
Unpaid Principal Balance	73	284
Related Allowance	0	0
Average Recorded Investment (Year-To-Date)	$ 87	$ 100