Allowance for Losses on Loans - Credit Risk Profile Based on Payment Activity (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
Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 11,821,138	$ 10,908,194
Total grade as a % of total net loans	99.50%	99.60%
Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 75,520	$ 71,441
Nonaccrual loans by portfolio segment	$ 56,408	$ 42,414
Grade as a percentage of total gross loans	0.50%	0.40%
Single-family residential
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 5,687,850	$ 5,658,830
Single-family residential | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 5,655,238	$ 5,625,682
Total grade as a % of total net loans	99.40%	99.40%
Single-family residential | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual loans by portfolio segment	$ 32,613	$ 33,148
Grade as a percentage of total gross loans	0.60%	0.60%
Construction
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 1,436,874	$ 1,110,411
Construction | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 1,436,873	$ 1,110,411
Total grade as a % of total net loans	100.00%	100.00%
Construction | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual loans by portfolio segment	$ 0	$ 0
Grade as a percentage of total gross loans	0.00%	0.00%
Construction - custom
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 561,260	$ 473,069
Construction - custom | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 560,724	$ 473,069
Total grade as a % of total net loans	99.90%	100.00%
Construction - custom | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual loans by portfolio segment	$ 536	$ 0
Grade as a percentage of total gross loans	0.10%	0.00%
Land - acquisition & development
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 119,524	$ 118,497
Land - acquisition & development | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 119,453	$ 118,439
Total grade as a % of total net loans	99.90%	99.90%
Land - acquisition & development | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual loans by portfolio segment	$ 71	$ 58
Grade as a percentage of total gross loans	0.10%	0.10%
Land - consumer lot loans
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 101,626	$ 104,567
Land - consumer lot loans | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 100,560	$ 104,057
Total grade as a % of total net loans	99.00%	99.50%
Land - consumer lot loans | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual loans by portfolio segment	$ 1,066	$ 510
Grade as a percentage of total gross loans	1.00%	0.50%
Multi-family
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 1,263,187	$ 1,124,290
Multi-family | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 1,262,505	$ 1,123,583
Total grade as a % of total net loans	99.90%	99.90%
Multi-family | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual loans by portfolio segment	$ 682	$ 776
Grade as a percentage of total gross loans	0.10%	0.10%
Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 1,346,006	$ 1,093,639
Commercial real estate | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 1,333,023	$ 1,086,470
Total grade as a % of total net loans	99.00%	99.30%
Commercial real estate | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual loans by portfolio segment	$ 12,983	$ 7,100
Grade as a percentage of total gross loans	1.00%	0.70%
Commercial & industrial
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 1,116,860	$ 978,589
Commercial & industrial | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 1,108,606	$ 978,006
Total grade as a % of total net loans	99.30%	99.90%
Commercial & industrial | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual loans by portfolio segment	$ 8,254	$ 583
Grade as a percentage of total gross loans	0.70%	0.10%
HELOC
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 148,584	$ 149,716
HELOC | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 148,403	$ 149,477
Total grade as a % of total net loans	99.90%	99.80%
HELOC | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual loans by portfolio segment	$ 181	$ 239
Grade as a percentage of total gross loans	0.10%	0.20%
Consumer
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 95,775	$ 139,000
Consumer | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 95,753	$ 139,000
Total grade as a % of total net loans	99.90%	100.00%
Consumer | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual loans by portfolio segment	$ 22	$ 0
Grade as a percentage of total gross loans	0.10%	0.00%