Loans Receivable - Loans Receivable, Analysis of Age of Loans in Past Due Status (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Sep. 30, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	$ 10,301,465	$ 10,071,124
Current	10,225,248	10,002,844
Past due	$ 76,217	$ 68,280
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.74%	0.68%
Ratio of total past due loans to total loans receivable, current	99.26%	99.32%
Ratio of total past due loans to total loans receivable, past due	0.74%	0.68%
Financing Receivables, 1 to 29 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	$ 23,060	$ 25,007
Ratio of total past due loans to total loans receivable, past due	0.22%	0.25%
Financing Receivables, 30 to 59 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	$ 8,323	$ 7,014
Ratio of total past due loans to total loans receivable, past due	0.08%	0.07%
Financing Receivables, 60 to 89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	$ 44,834	$ 36,259
Ratio of total past due loans to total loans receivable, past due	0.44%	0.36%
Single-family residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	$ 5,623,668	$ 5,658,122
Current	5,563,465	5,601,457
Past due	$ 60,203	$ 56,665
Ratio of past due loans to total loans, net of charge-offs and LIPs	1.07%	1.00%
Single-family residential | Financing Receivables, 1 to 29 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	$ 16,670	$ 20,916
Single-family residential | Financing Receivables, 30 to 59 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	6,808	5,271
Single-family residential | Financing Receivables, 60 to 89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	36,725	30,478
Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	513,046	498,450
Current	512,705	498,450
Past due	$ 341	$ 0
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.07%	0.00%
Construction | Financing Receivables, 1 to 29 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	$ 341	$ 0
Construction | Financing Receivables, 30 to 59 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	0	0
Construction | Financing Receivables, 60 to 89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	0	0
Construction - custom
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	236,668	229,957
Current	236,511	229,419
Past due	$ 157	$ 538
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.07%	0.23%
Construction - custom | Financing Receivables, 1 to 29 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	$ 49	$ 538
Construction - custom | Financing Receivables, 30 to 59 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	108	0
Construction - custom | Financing Receivables, 60 to 89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	0	0
Land - acquisition & development
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	103,148	94,928
Current	101,886	94,928
Past due	$ 1,262	$ 0
Ratio of past due loans to total loans, net of charge-offs and LIPs	1.22%	0.00%
Land - acquisition & development | Financing Receivables, 1 to 29 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	$ 728	$ 0
Land - acquisition & development | Financing Receivables, 30 to 59 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	0	0
Land - acquisition & development | Financing Receivables, 60 to 89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	534	0
Land - consumer lot loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	101,045	104,534
Current	99,969	102,472
Past due	$ 1,076	$ 2,062
Ratio of past due loans to total loans, net of charge-offs and LIPs	1.06%	1.97%
Land - consumer lot loans | Financing Receivables, 1 to 29 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	$ 235	$ 816
Land - consumer lot loans | Financing Receivables, 30 to 59 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	43	687
Land - consumer lot loans | Financing Receivables, 60 to 89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	798	559
Multi-family
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	1,217,594	1,124,290
Current	1,215,726	1,122,307
Past due	$ 1,868	$ 1,983
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.15%	0.18%
Multi-family | Financing Receivables, 1 to 29 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	$ 853	$ 1,190
Multi-family | Financing Receivables, 30 to 59 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	616	399
Multi-family | Financing Receivables, 60 to 89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	399	394
Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	1,175,475	1,093,549
Current	1,167,710	1,088,680
Past due	$ 7,765	$ 4,869
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.66%	0.45%
Commercial real estate | Financing Receivables, 1 to 29 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	$ 1,771	$ 69
Commercial real estate | Financing Receivables, 30 to 59 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	267	325
Commercial real estate | Financing Receivables, 60 to 89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	5,727	4,475
Commercial & industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	1,057,826	978,582
Current	1,056,718	978,540
Past due	$ 1,108	$ 42
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.10%	0.00%
Commercial & industrial | Financing Receivables, 1 to 29 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	$ 858	$ 0
Commercial & industrial | Financing Receivables, 30 to 59 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	250	42
Commercial & industrial | Financing Receivables, 60 to 89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	0	0
HELOC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	148,448	149,713
Current	147,074	148,513
Past due	$ 1,374	$ 1,200
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.93%	0.80%
HELOC | Financing Receivables, 1 to 29 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	$ 893	$ 763
HELOC | Financing Receivables, 30 to 59 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	0	164
HELOC | Financing Receivables, 60 to 89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	481	273
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	124,547	138,999
Current	123,484	138,078
Past due	$ 1,063	$ 921
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.85%	0.66%
Consumer | Financing Receivables, 1 to 29 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	$ 662	$ 715
Consumer | Financing Receivables, 30 to 59 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	231	126
Consumer | Financing Receivables, 60 to 89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	$ 170	$ 80