Allowance for Losses on Loans - Impaired Loans Based on Loan Types (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Mar. 31, 2020	Sep. 30, 2019
Financing Receivable, Impaired [Line Items]
Recorded Investment	$ 132,925	$ 149,546
Unpaid Principal Balance	140,917	164,561
Related Allowance	1,106	2,647
Average Recorded Investment (Year-To-Date)	139,377	175,187
Amount of related allowance included in specific reserves	9	439
Amount of related allowance included in general reserves	1,097	2,208
Impaired loans with no related allowance recorded
Financing Receivable, Impaired [Line Items]
Recorded Investment	28,906	27,869
Unpaid Principal Balance	34,240	38,486
Related Allowance	0	0
Average Recorded Investment (Year-To-Date)	27,202	35,649
Impaired loans with an allowance recorded
Financing Receivable, Impaired [Line Items]
Recorded Investment	104,019	121,677
Unpaid Principal Balance	106,677	126,075
Related Allowance	1,106	2,647
Average Recorded Investment (Year-To-Date)	112,175	139,538
Single-family residential
Financing Receivable, Impaired [Line Items]
Recorded Investment	113,547	130,021
Unpaid Principal Balance	116,757	133,861
Related Allowance	1,097	2,208
Average Recorded Investment (Year-To-Date)	120,745	142,661
Single-family residential | Impaired loans with no related allowance recorded
Financing Receivable, Impaired [Line Items]
Recorded Investment	16,523	17,979
Unpaid Principal Balance	17,440	19,252
Related Allowance	0	0
Average Recorded Investment (Year-To-Date)	17,002	16,685
Single-family residential | Impaired loans with an allowance recorded
Financing Receivable, Impaired [Line Items]
Recorded Investment	97,024	112,042
Unpaid Principal Balance	99,317	114,609
Related Allowance	1,097	2,208
Average Recorded Investment (Year-To-Date)	103,743	125,976
Construction
Financing Receivable, Impaired [Line Items]
Recorded Investment	3,434	0
Unpaid Principal Balance	3,434	0
Related Allowance	0	0
Average Recorded Investment (Year-To-Date)	1,145	1,172
Construction | Impaired loans with no related allowance recorded
Financing Receivable, Impaired [Line Items]
Recorded Investment	3,434	0
Unpaid Principal Balance	3,434	0
Related Allowance	0	0
Average Recorded Investment (Year-To-Date)	1,145	1,172
Construction - custom
Financing Receivable, Impaired [Line Items]
Recorded Investment		0
Unpaid Principal Balance		0
Related Allowance		0
Average Recorded Investment (Year-To-Date)		251
Construction - custom | Impaired loans with no related allowance recorded
Financing Receivable, Impaired [Line Items]
Recorded Investment		0
Unpaid Principal Balance		0
Related Allowance		0
Average Recorded Investment (Year-To-Date)		251
Land - acquisition & development
Financing Receivable, Impaired [Line Items]
Recorded Investment	82	169
Unpaid Principal Balance	149	295
Related Allowance	1	0
Average Recorded Investment (Year-To-Date)	112	389
Land - acquisition & development | Impaired loans with no related allowance recorded
Financing Receivable, Impaired [Line Items]
Recorded Investment	0	78
Unpaid Principal Balance	0	143
Related Allowance	0	0
Average Recorded Investment (Year-To-Date)	26	290
Land - acquisition & development | Impaired loans with an allowance recorded
Financing Receivable, Impaired [Line Items]
Recorded Investment	82	91
Unpaid Principal Balance	149	152
Related Allowance	1	0
Average Recorded Investment (Year-To-Date)	86	99
Land - consumer lot loans
Financing Receivable, Impaired [Line Items]
Recorded Investment	3,698	3,900
Unpaid Principal Balance	3,933	4,543
Related Allowance	0	20
Average Recorded Investment (Year-To-Date)	3,808	4,611
Land - consumer lot loans | Impaired loans with no related allowance recorded
Financing Receivable, Impaired [Line Items]
Recorded Investment	280	344
Unpaid Principal Balance	350	848
Related Allowance	0	0
Average Recorded Investment (Year-To-Date)	298	287
Land - consumer lot loans | Impaired loans with an allowance recorded
Financing Receivable, Impaired [Line Items]
Recorded Investment	3,418	3,556
Unpaid Principal Balance	3,583	3,695
Related Allowance	0	20
Average Recorded Investment (Year-To-Date)	3,510	4,324
Multi-family
Financing Receivable, Impaired [Line Items]
Recorded Investment	316	385
Unpaid Principal Balance	316	385
Related Allowance	1	4
Average Recorded Investment (Year-To-Date)	360	704
Multi-family | Impaired loans with no related allowance recorded
Financing Receivable, Impaired [Line Items]
Recorded Investment		0
Unpaid Principal Balance		0
Related Allowance		0
Average Recorded Investment (Year-To-Date)		286
Multi-family | Impaired loans with an allowance recorded
Financing Receivable, Impaired [Line Items]
Recorded Investment	316	385
Unpaid Principal Balance	316	385
Related Allowance	1	4
Average Recorded Investment (Year-To-Date)	360	418
Commercial real estate
Financing Receivable, Impaired [Line Items]
Recorded Investment	9,295	11,635
Unpaid Principal Balance	13,636	17,179
Related Allowance	4	323
Average Recorded Investment (Year-To-Date)	10,472	14,050
Commercial real estate | Impaired loans with no related allowance recorded
Financing Receivable, Impaired [Line Items]
Recorded Investment	7,412	7,467
Unpaid Principal Balance	11,753	11,881
Related Allowance	0	0
Average Recorded Investment (Year-To-Date)	7,376	8,890
Commercial real estate | Impaired loans with an allowance recorded
Financing Receivable, Impaired [Line Items]
Recorded Investment	1,883	4,168
Unpaid Principal Balance	1,883	5,298
Related Allowance	4	323
Average Recorded Investment (Year-To-Date)	3,096	5,160
Commercial & industrial
Financing Receivable, Impaired [Line Items]
Recorded Investment	855	1,540
Unpaid Principal Balance	978	6,003
Related Allowance	3	92
Average Recorded Investment (Year-To-Date)	1,081	9,703
Commercial & industrial | Impaired loans with no related allowance recorded
Financing Receivable, Impaired [Line Items]
Recorded Investment	458	1,114
Unpaid Principal Balance	464	5,312
Related Allowance	0	0
Average Recorded Investment (Year-To-Date)	670	7,168
Commercial & industrial | Impaired loans with an allowance recorded
Financing Receivable, Impaired [Line Items]
Recorded Investment	397	426
Unpaid Principal Balance	514	691
Related Allowance	3	92
Average Recorded Investment (Year-To-Date)	411	2,535
HELOC
Financing Receivable, Impaired [Line Items]
Recorded Investment	1,642	1,786
Unpaid Principal Balance	1,658	1,894
Related Allowance	0	0
Average Recorded Investment (Year-To-Date)	1,579	1,558
HELOC | Impaired loans with no related allowance recorded
Financing Receivable, Impaired [Line Items]
Recorded Investment	799	837
Unpaid Principal Balance	799	931
Related Allowance	0	0
Average Recorded Investment (Year-To-Date)	668	597
HELOC | Impaired loans with an allowance recorded
Financing Receivable, Impaired [Line Items]
Recorded Investment	843	949
Unpaid Principal Balance	859	963
Related Allowance	0	0
Average Recorded Investment (Year-To-Date)	911	961
Consumer
Financing Receivable, Impaired [Line Items]
Recorded Investment	56	110
Unpaid Principal Balance	56	401
Related Allowance	0	0
Average Recorded Investment (Year-To-Date)	75	88
Consumer | Impaired loans with no related allowance recorded
Financing Receivable, Impaired [Line Items]
Recorded Investment	0	50
Unpaid Principal Balance	0	119
Related Allowance	0	0
Average Recorded Investment (Year-To-Date)	17	23
Consumer | Impaired loans with an allowance recorded
Financing Receivable, Impaired [Line Items]
Recorded Investment	56	60
Unpaid Principal Balance	56	282
Related Allowance	0	0
Average Recorded Investment (Year-To-Date)	$ 58	$ 65