Loans Receivable - Loans Receivable, Analysis of Age of Loans in Past Due Status (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Sep. 30, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	$ 10,823,033	$ 10,071,124
Current	10,768,633	10,002,844
Past due	$ 54,401	$ 68,280
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.50%	0.68%
Ratio of total past due loans to total loans receivable, current	99.50%	99.32%
Ratio of total past due loans to total loans receivable, past due	0.50%	0.68%
Financing Receivables, 1 to 29 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	$ 12,481	$ 25,007
Ratio of total past due loans to total loans receivable, past due	0.12%	0.25%
Financing Receivables, 30 to 59 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	$ 11,150	$ 7,014
Ratio of total past due loans to total loans receivable, past due	0.10%	0.07%
Financing Receivables, 60 to 89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	$ 30,770	$ 36,259
Ratio of total past due loans to total loans receivable, past due	0.28%	0.36%
Single-family residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	$ 5,687,169	$ 5,658,122
Current	5,642,516	5,601,457
Past due	$ 44,653	$ 56,665
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.79%	1.00%
Single-family residential | Financing Receivables, 1 to 29 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	$ 9,307	$ 20,916
Single-family residential | Financing Receivables, 30 to 59 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	7,004	5,271
Single-family residential | Financing Receivables, 60 to 89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	28,342	30,478
Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	683,273	498,450
Current	682,587	498,450
Past due	$ 686	$ 0
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.10%	0.00%
Construction | Financing Receivables, 1 to 29 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	$ 0	$ 0
Construction | Financing Receivables, 30 to 59 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	686	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	269,612	229,957
Current	269,076	229,419
Past due	$ 536	$ 538
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.20%	0.23%
Construction - custom | Financing Receivables, 1 to 29 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	$ 0	$ 538
Construction - custom | Financing Receivables, 30 to 59 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	0	0
Construction - custom | Financing Receivables, 60 to 89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	536	0
Land - acquisition & development
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	111,057	94,928
Current	110,934	94,928
Past due	$ 123	$ 0
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.11%	0.00%
Land - acquisition & development | Financing Receivables, 1 to 29 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	$ 0	$ 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	123	0
Land - consumer lot loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	101,584	104,534
Current	100,969	102,472
Past due	$ 615	$ 2,062
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.61%	1.97%
Land - consumer lot loans | Financing Receivables, 1 to 29 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	$ 36	$ 816
Land - consumer lot loans | Financing Receivables, 30 to 59 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	300	687
Land - consumer lot loans | Financing Receivables, 60 to 89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	279	559
Multi-family
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	1,263,143	1,124,290
Current	1,262,814	1,122,307
Past due	$ 329	$ 1,983
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.03%	0.18%
Multi-family | Financing Receivables, 1 to 29 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	$ 139	$ 1,190
Multi-family | Financing Receivables, 30 to 59 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	190	399
Multi-family | Financing Receivables, 60 to 89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	0	394
Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	1,345,986	1,093,549
Current	1,343,075	1,088,680
Past due	$ 2,911	$ 4,869
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.22%	0.45%
Commercial real estate | Financing Receivables, 1 to 29 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	$ 325	$ 69
Commercial real estate | Financing Receivables, 30 to 59 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	1,728	325
Commercial real estate | Financing Receivables, 60 to 89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	858	4,475
Commercial & industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	1,116,854	978,582
Current	1,114,182	978,540
Past due	$ 2,673	$ 42
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.24%	0.00%
Commercial & industrial | Financing Receivables, 1 to 29 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	$ 1,599	$ 0
Commercial & industrial | Financing Receivables, 30 to 59 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	500	42
Commercial & industrial | Financing Receivables, 60 to 89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	574	0
HELOC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	148,581	149,713
Current	147,090	148,513
Past due	$ 1,491	$ 1,200
Ratio of past due loans to total loans, net of charge-offs and LIPs	1.00%	0.80%
HELOC | Financing Receivables, 1 to 29 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	$ 808	$ 763
HELOC | Financing Receivables, 30 to 59 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	647	164
HELOC | Financing Receivables, 60 to 89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	36	273
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	95,774	138,999
Current	95,390	138,078
Past due	$ 384	$ 921
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.40%	0.66%
Consumer | Financing Receivables, 1 to 29 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	$ 267	$ 715
Consumer | Financing Receivables, 30 to 59 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	95	126
Consumer | Financing Receivables, 60 to 89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	$ 22	$ 80