Allowance for Losses on Loans - Loans Evaluated for Impairment (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	$ 118,456	$ 113,494	$ 107,901	$ 106,829
Loans receivable, gross	11,328,633	10,950,608
Loans Collectively Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	118,089	113,128
Loans receivable, gross	$ 10,171,700	$ 9,784,140
Ratio	1.20%	1.20%
Loans Individually Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	$ 367	$ 366
Loans receivable, gross	$ 51,726	$ 34,271
Ratio	0.70%	1.10%
Single-family residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	$ 38,206	$ 37,796	47,756	47,347
Loans receivable, gross	5,624,263	5,658,830
Single-family residential | Loans Collectively Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	38,092	37,536
Loans receivable, gross	$ 5,585,461	$ 5,585,912
Ratio	0.70%	0.70%
Single-family residential | Loans Individually Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	$ 114	$ 260
Loans receivable, gross	$ 30,165	$ 19,629
Ratio	0.40%	1.30%
Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	$ 21,934	$ 19,838	7,014	6,680
Loans receivable, gross	1,265,747	1,110,411
Construction | Loans Collectively Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	21,934	19,838
Loans receivable, gross	$ 513,046	$ 498,450
Ratio	4.30%	4.00%
Construction | Loans Individually Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	$ 0	$ 0
Loans receivable, gross	$ 0	$ 0
Ratio	0.00%	0.00%
Construction - custom
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	$ 1,110	$ 1,080	1,062	990
Loans receivable, gross	494,447	473,069
Construction - custom | Loans Collectively Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	1,110	1,080
Loans receivable, gross	$ 236,668	$ 229,298
Ratio	0.50%	0.50%
Construction - custom | Loans Individually Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	$ 0	$ 0
Loans receivable, gross	$ 0	$ 330
Ratio	0.00%	0.00%
Land - acquisition & development
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	$ 6,665	$ 6,023	6,778	5,781
Loans receivable, gross	119,085	118,497
Land - acquisition & development | Loans Collectively Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	6,621	6,022
Loans receivable, gross	$ 101,598	$ 90,850
Ratio	6.50%	6.60%
Land - acquisition & development | Loans Individually Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	$ 44	$ 2
Loans receivable, gross	$ 886	$ 850
Ratio	5.00%	0.20%
Land - consumer lot loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	$ 2,501	$ 2,535	3,001	2,946
Loans receivable, gross	101,104	104,567
Land - consumer lot loans | Loans Collectively Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	2,501	2,535
Loans receivable, gross	$ 90,947	$ 92,828
Ratio	2.70%	2.70%
Land - consumer lot loans | Loans Individually Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	$ 0	$ 0
Loans receivable, gross	$ 1,384	$ 558
Ratio	0.00%	0.00%
Multi-family
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	$ 7,629	$ 6,925	5,047	5,304
Loans receivable, gross	1,217,594	1,124,290
Multi-family | Loans Collectively Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	7,617	6,911
Loans receivable, gross	$ 1,215,870	$ 1,091,974
Ratio	0.60%	0.60%
Multi-family | Loans Individually Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	$ 12	$ 13
Loans receivable, gross	$ 1,496	$ 1,505
Ratio	0.80%	0.90%
Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	$ 10,168	$ 8,588	10,344	8,960
Loans receivable, gross	1,207,573	1,093,639
Commercial real estate | Loans Collectively Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	9,971	8,497
Loans receivable, gross	$ 1,144,389	$ 957,380
Ratio	0.90%	0.90%
Commercial real estate | Loans Individually Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	$ 197	$ 91
Loans receivable, gross	$ 15,473	$ 11,157
Ratio	1.30%	0.80%
Commercial & industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	$ 27,736	$ 28,008	24,096	24,980
Loans receivable, gross	1,025,821	978,589
Commercial & industrial | Loans Collectively Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	27,736	28,008
Loans receivable, gross	$ 1,021,372	$ 966,930
Ratio	2.70%	2.90%
Commercial & industrial | Loans Individually Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	$ 0	$ 0
Loans receivable, gross	$ 1,517	$ 0
Ratio	0.00%	0.00%
HELOC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	$ 832	$ 813	820	902
Loans receivable, gross	148,452	149,716
HELOC | Loans Collectively Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	832	813
Loans receivable, gross	$ 138,094	$ 133,203
Ratio	0.60%	0.60%
HELOC | Loans Individually Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	$ 0	$ 0
Loans receivable, gross	$ 805	$ 239
Ratio	0.00%	0.00%
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	$ 1,675	$ 1,888	$ 1,983	$ 2,939
Loans receivable, gross	124,547	139,000
Consumer | Loans Collectively Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	1,675	1,888
Loans receivable, gross	$ 124,255	$ 137,315
Ratio	1.30%	1.40%
Consumer | Loans Individually Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	$ 0	$ 0
Loans receivable, gross	$ 0	$ 3
Ratio	0.00%	0.00%