Allowance for Losses on Loans - Credit Risk Profile Based on Payment Activity (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Sep. 30, 2018
Financing Receivable, Recorded Investment [Line Items]
Performing Loans, Amount	$ 13,304,743	$ 12,853,678
Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Performing Loans, Amount	$ 13,263,738	$ 12,797,992
Performing Loans, % of Total Gross Loans	99.70%	99.60%
Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Performing Loans, Amount	$ 51,117	$ 70,093
Non-Performing Loans, Amount	$ 41,005	$ 55,686
Non-Performing Loans, % of Total Gross Loans	0.30%	0.40%
Single-family residential
Financing Receivable, Recorded Investment [Line Items]
Performing Loans, Amount	$ 5,873,583	$ 5,798,966
Single-family residential | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Performing Loans, Amount	$ 5,851,298	$ 5,771,323
Performing Loans, % of Total Gross Loans	99.60%	99.50%
Single-family residential | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Non-Performing Loans, Amount	$ 22,285	$ 27,643
Non-Performing Loans, % of Total Gross Loans	0.40%	0.50%
Construction
Financing Receivable, Recorded Investment [Line Items]
Performing Loans, Amount	$ 1,997,236	$ 1,890,668
Construction | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Performing Loans, Amount	$ 1,997,236	$ 1,888,241
Performing Loans, % of Total Gross Loans	100.00%	99.90%
Construction | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Non-Performing Loans, Amount	$ 0	$ 2,427
Non-Performing Loans, % of Total Gross Loans	0.00%	0.10%
Construction - custom
Financing Receivable, Recorded Investment [Line Items]
Performing Loans, Amount	$ 570,897	$ 624,479
Construction - custom | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Performing Loans, Amount	$ 569,736	$ 624,479
Performing Loans, % of Total Gross Loans	99.80%	100.00%
Construction - custom | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Non-Performing Loans, Amount	$ 1,161	$ 0
Non-Performing Loans, % of Total Gross Loans	0.20%	0.00%
Land - acquisition & development
Financing Receivable, Recorded Investment [Line Items]
Performing Loans, Amount	$ 203,086	$ 155,204
Land - acquisition & development | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Performing Loans, Amount	$ 202,913	$ 154,284
Performing Loans, % of Total Gross Loans	99.90%	99.40%
Land - acquisition & development | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Non-Performing Loans, Amount	$ 173	$ 920
Non-Performing Loans, % of Total Gross Loans	0.10%	0.60%
Land - consumer lot loans
Financing Receivable, Recorded Investment [Line Items]
Performing Loans, Amount	$ 95,227	$ 102,036
Land - consumer lot loans | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Performing Loans, Amount	$ 94,586	$ 101,249
Performing Loans, % of Total Gross Loans	99.30%	99.20%
Land - consumer lot loans | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Non-Performing Loans, Amount	$ 641	$ 787
Non-Performing Loans, % of Total Gross Loans	0.70%	0.80%
Multi-family
Financing Receivable, Recorded Investment [Line Items]
Performing Loans, Amount	$ 1,403,142	$ 1,385,125
Multi-family | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Performing Loans, Amount	$ 1,401,711	$ 1,385,125
Performing Loans, % of Total Gross Loans	99.90%	100.00%
Multi-family | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Non-Performing Loans, Amount	$ 1,431	$ 0
Non-Performing Loans, % of Total Gross Loans	0.10%	0.00%
Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Performing Loans, Amount	$ 1,622,943	$ 1,452,168
Commercial real estate | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Performing Loans, Amount	$ 1,614,479	$ 1,443,197
Performing Loans, % of Total Gross Loans	99.50%	99.40%
Commercial real estate | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Non-Performing Loans, Amount	$ 8,464	$ 8,971
Non-Performing Loans, % of Total Gross Loans	0.50%	0.60%
Commercial & industrial
Financing Receivable, Recorded Investment [Line Items]
Performing Loans, Amount	$ 1,256,398	$ 1,140,874
Commercial & industrial | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Performing Loans, Amount	$ 1,250,351	$ 1,126,480
Performing Loans, % of Total Gross Loans	99.50%	98.70%
Commercial & industrial | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Non-Performing Loans, Amount	$ 6,047	$ 14,394
Non-Performing Loans, % of Total Gross Loans	0.50%	1.30%
HELOC
Financing Receivable, Recorded Investment [Line Items]
Performing Loans, Amount	$ 139,914	$ 130,852
HELOC | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Performing Loans, Amount	$ 139,111	$ 130,329
Performing Loans, % of Total Gross Loans	99.40%	99.60%
HELOC | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Non-Performing Loans, Amount	$ 803	$ 523
Non-Performing Loans, % of Total Gross Loans	0.60%	0.40%
Consumer
Financing Receivable, Recorded Investment [Line Items]
Performing Loans, Amount	$ 142,317	$ 173,306
Consumer | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Performing Loans, Amount	$ 142,317	$ 173,285
Performing Loans, % of Total Gross Loans	100.00%	100.00%
Consumer | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Non-Performing Loans, Amount	$ 0	$ 21
Non-Performing Loans, % of Total Gross Loans	0.00%	0.00%