Loans Receivable - Loans Receivable, Analysis of Age of Loans in Past Due Status (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Sep. 30, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Net of Loans In Process	$ 11,507,160	$ 11,051,453
Current	11,461,528	11,007,440
Total Delinquent	$ 45,632	$ 44,013
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.40%	0.40%
Ratio of total past due loans to total loans receivable, current	99.60%	99.60%
Ratio of total past due loans to total loans receivable, past due	0.40%	0.40%
30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Delinquent	$ 9,172	$ 12,015
Ratio of total past due loans to total loans receivable, past due	0.08%	0.11%
60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Delinquent	$ 5,981	$ 6,245
Ratio of total past due loans to total loans receivable, past due	0.05%	0.06%
90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Delinquent	$ 30,479	$ 25,753
Ratio of total past due loans to total loans receivable, past due	0.26%	0.23%
Single-family residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Net of Loans In Process	$ 5,744,620	$ 5,709,690
Current	5,714,962	5,671,933
Total Delinquent	$ 29,658	$ 37,757
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.52%	0.66%
Single-family residential | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Delinquent	$ 7,201	$ 10,925
Single-family residential | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Delinquent	4,521	4,810
Single-family residential | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Delinquent	17,936	22,022
Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Net of Loans In Process	1,012,239	793,959
Current	1,009,782	793,959
Total Delinquent	$ 2,457	$ 0
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.24%	0.00%
Construction | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Delinquent	$ 0	$ 0
Construction | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Delinquent	616	0
Construction | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Delinquent	1,841	0
Construction - custom
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Net of Loans In Process	285,858	277,599
Current	285,687	277,508
Total Delinquent	$ 171	$ 91
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.06%	0.03%
Construction - custom | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Delinquent	$ 171	$ 0
Construction - custom | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Delinquent	0	0
Construction - custom | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Delinquent	0	91
Land - acquisition & development
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Net of Loans In Process	121,508	104,856
Current	119,593	104,526
Total Delinquent	$ 1,915	$ 330
Ratio of past due loans to total loans, net of charge-offs and LIPs	1.58%	0.31%
Land - acquisition & development | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Delinquent	$ 0	$ 0
Land - acquisition & development | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Delinquent	270	0
Land - acquisition & development | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Delinquent	1,645	330
Land - consumer lot loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Net of Loans In Process	103,040	104,335
Current	102,303	103,389
Total Delinquent	$ 737	$ 946
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.72%	0.91%
Land - consumer lot loans | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Delinquent	$ 128	$ 112
Land - consumer lot loans | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Delinquent	301	680
Land - consumer lot loans | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Delinquent	308	154
Multi-family
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Net of Loans In Process	1,346,512	1,303,119
Current	1,346,512	1,302,720
Total Delinquent	$ 0	$ 399
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.00%	0.03%
Multi-family | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Delinquent	$ 0	$ 5
Multi-family | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Delinquent	0	255
Multi-family | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Delinquent	0	139
Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Net of Loans In Process	1,435,417	1,434,610
Current	1,433,030	1,432,052
Total Delinquent	$ 2,387	$ 2,558
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.17%	0.18%
Commercial real estate | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Delinquent	$ 684	$ 507
Commercial real estate | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Delinquent	0	0
Commercial real estate | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Delinquent	1,703	2,051
Commercial & industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Net of Loans In Process	1,133,075	1,093,360
Current	1,126,480	1,092,735
Total Delinquent	$ 6,595	$ 625
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.58%	0.06%
Commercial & industrial | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Delinquent	$ 0	$ 0
Commercial & industrial | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Delinquent	0	51
Commercial & industrial | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Delinquent	6,595	574
HELOC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Net of Loans In Process	136,766	144,850
Current	135,406	143,974
Total Delinquent	$ 1,360	$ 876
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.99%	0.60%
HELOC | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Delinquent	$ 863	$ 221
HELOC | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Delinquent	146	342
HELOC | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Delinquent	351	313
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Net of Loans In Process	188,125	85,075
Current	187,773	84,644
Total Delinquent	$ 352	$ 431
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.19%	0.51%
Consumer | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Delinquent	$ 125	$ 245
Consumer | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Delinquent	127	107
Consumer | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Delinquent	$ 100	$ 79