Allowance for Losses on Loans - Credit Risk Profile Based on Payment Activity (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Sep. 30, 2016
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 11,328,633	$ 10,950,608
Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 11,045,267	$ 10,673,989
Total grade as a % of total net loans	99.50%	99.60%
Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 83,352	$ 71,441
Nonaccrual loans by portfolio segment	$ 60,715	$ 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	$ 5,624,263	$ 5,658,830
Single-family residential | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 5,549,864	$ 5,587,919
Total grade as a % of total net loans	99.30%	99.40%
Single-family residential | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual loans by portfolio segment	$ 38,568	$ 33,148
Grade as a percentage of total gross loans	0.70%	0.60%
Construction
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 1,265,747	$ 1,110,411
Construction | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 1,265,747	$ 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	$ 494,447	$ 473,069
Construction - custom | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 494,447	$ 473,069
Total grade as a % of total net loans	100.00%	100.00%
Construction - custom | 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%
Land - acquisition & development
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 119,085	$ 118,497
Land - acquisition & development | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 116,567	$ 116,097
Total grade as a % of total net loans	99.50%	99.90%
Land - acquisition & development | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual loans by portfolio segment	$ 603	$ 58
Grade as a percentage of total gross loans	0.50%	0.10%
Land - consumer lot loans
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 101,104	$ 104,567
Land - consumer lot loans | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 97,556	$ 101,343
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	$ 969	$ 510
Grade as a percentage of total gross loans	1.00%	0.50%
Multi-family
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 1,217,594	$ 1,124,290
Multi-family | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 1,211,411	$ 1,118,025
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	$ 1,160	$ 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	$ 1,207,573	$ 1,093,639
Commercial real estate | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 1,079,298	$ 949,064
Total grade as a % of total net loans	99.10%	99.30%
Commercial real estate | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual loans by portfolio segment	$ 9,660	$ 7,100
Grade as a percentage of total gross loans	0.90%	0.70%
Commercial & industrial
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 1,025,821	$ 978,589
Commercial & industrial | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 971,931	$ 946,065
Total grade as a % of total net loans	99.10%	99.90%
Commercial & industrial | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual loans by portfolio segment	$ 9,230	$ 583
Grade as a percentage of total gross loans	0.90%	0.10%
HELOC
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 148,452	$ 149,716
HELOC | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 135,306	$ 134,546
Total grade as a % of total net loans	99.60%	99.80%
HELOC | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual loans by portfolio segment	$ 480	$ 239
Grade as a percentage of total gross loans	0.40%	0.20%
Consumer
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 124,547	$ 139,000
Consumer | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 123,140	$ 137,450
Total grade as a % of total net loans	100.00%	100.00%
Consumer | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual loans by portfolio segment	$ 45	$ 0
Grade as a percentage of total gross loans	0.00%	0.00%