Allowance for Losses on Loans - Internally Assigned Grade (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Sep. 30, 2016
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 11,877,546	$ 10,950,608
Pass
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 11,729,048	$ 10,764,097
Total grade as a % of total net loans	98.70%	98.30%
Special mention
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 34,027	$ 32,485
Total grade as a % of total net loans	0.30%	0.30%
Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 114,471	$ 154,026
Total grade as a % of total net loans	1.00%	1.40%
Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 0	$ 0
Total grade as a % of total net loans	0.00%	0.00%
Loss
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 0	$ 0
Total grade as a % of total net loans	0.00%	0.00%
Single-family residential
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 5,687,850	$ 5,658,830
Single-family residential | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	5,642,507	5,607,521
Single-family residential | Special mention
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Single-family residential | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	45,343	51,309
Single-family residential | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Single-family residential | Loss
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Construction
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	1,436,874	1,110,411
Construction | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	1,427,443	1,098,549
Construction | Special mention
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	5,876	8,595
Construction | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	3,555	3,267
Construction | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Construction | Loss
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Construction - custom
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	561,260	473,069
Construction - custom | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	560,724	473,069
Construction - custom | Special mention
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Construction - custom | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	536	0
Construction - custom | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Construction - custom | Loss
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Land - acquisition & development
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	119,524	118,497
Land - acquisition & development | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	117,004	111,225
Land - acquisition & development | Special mention
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	207	0
Land - acquisition & development | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	2,313	7,272
Land - acquisition & development | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Land - acquisition & development | Loss
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Land - consumer lot loans
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	101,626	104,567
Land - consumer lot loans | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	100,436	103,528
Land - consumer lot loans | Special mention
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Land - consumer lot loans | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	1,190	1,039
Land - consumer lot loans | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Land - consumer lot loans | Loss
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Multi-family
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	1,263,187	1,124,290
Multi-family | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	1,248,315	1,117,437
Multi-family | Special mention
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	3,191	3,237
Multi-family | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	11,681	3,616
Multi-family | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Multi-family | Loss
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	1,346,006	1,093,639
Commercial real estate | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	1,305,769	1,033,880
Commercial real estate | Special mention
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	4,303	13,446
Commercial real estate | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	35,934	46,313
Commercial real estate | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Commercial real estate | Loss
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Commercial & industrial
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	1,116,860	978,589
Commercial & industrial | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	1,083,039	930,776
Commercial & industrial | Special mention
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	20,450	7,207
Commercial & industrial | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	13,371	40,606
Commercial & industrial | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Commercial & industrial | Loss
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
HELOC
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	148,584	149,716
HELOC | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	148,063	149,195
HELOC | Special mention
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
HELOC | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	521	521
HELOC | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
HELOC | Loss
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Consumer
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	95,775	139,000
Consumer | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	95,748	138,917
Consumer | Special mention
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Consumer | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	27	83
Consumer | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Consumer | Loss
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 0	$ 0