Allowance for Losses on Loans - Credit Risk Profile Based on Payment Activity (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Sep. 30, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
Performing Loans, Amount	$ 13,434,649	$ 13,312,388
Performing Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Performing Loans, Amount	$ 13,402,039	$ 13,278,657
Performing Loans, % of Total Gross Loans	99.80%	99.70%
Non-Performing Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Performing Loans, Amount	$ 41,387	$ 43,826
Non-Performing Loans, Amount	$ 32,610	$ 33,731
Non-Performing Loans, % of Total Gross Loans	0.20%	0.30%
Single-family residential
Financing Receivable, Credit Quality Indicator [Line Items]
Performing Loans, Amount	$ 5,582,244	$ 5,835,194
Single-family residential | Performing Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Performing Loans, Amount	$ 5,559,385	$ 5,809,923
Performing Loans, % of Total Gross Loans	99.60%	99.60%
Single-family residential | Non-Performing Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Non-Performing Loans, Amount	$ 22,859	$ 25,271
Non-Performing Loans, % of Total Gross Loans	0.40%	0.40%
Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Performing Loans, Amount	$ 2,204,283	$ 2,038,052
Construction | Performing Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Performing Loans, Amount	$ 2,200,930	$ 2,038,052
Performing Loans, % of Total Gross Loans	99.80%	100.00%
Construction | Non-Performing Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Non-Performing Loans, Amount	$ 3,353	$ 0
Non-Performing Loans, % of Total Gross Loans	0.20%	0.00%
Construction - custom
Financing Receivable, Credit Quality Indicator [Line Items]
Performing Loans, Amount	$ 547,731	$ 540,741
Construction - custom | Performing Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Performing Loans, Amount	$ 547,731	$ 540,741
Performing Loans, % of Total Gross Loans	100.00%	100.00%
Construction - custom | Non-Performing Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Non-Performing Loans, Amount	$ 0	$ 0
Non-Performing Loans, % of Total Gross Loans	0.00%	0.00%
Land - acquisition & development
Financing Receivable, Credit Quality Indicator [Line Items]
Performing Loans, Amount	$ 197,010	$ 204,107
Land - acquisition & development | Performing Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Performing Loans, Amount	$ 196,928	$ 203,938
Performing Loans, % of Total Gross Loans	100.00%	99.90%
Land - acquisition & development | Non-Performing Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Non-Performing Loans, Amount	$ 82	$ 169
Non-Performing Loans, % of Total Gross Loans	0.00%	0.10%
Land - consumer lot loans
Financing Receivable, Credit Quality Indicator [Line Items]
Performing Loans, Amount	$ 96,579	$ 99,694
Land - consumer lot loans | Performing Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Performing Loans, Amount	$ 96,171	$ 99,448
Performing Loans, % of Total Gross Loans	99.60%	99.80%
Land - consumer lot loans | Non-Performing Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Non-Performing Loans, Amount	$ 408	$ 246
Non-Performing Loans, % of Total Gross Loans	0.40%	0.20%
Multi-family
Financing Receivable, Credit Quality Indicator [Line Items]
Performing Loans, Amount	$ 1,467,231	$ 1,422,674
Multi-family | Performing Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Performing Loans, Amount	$ 1,467,231	$ 1,422,674
Performing Loans, % of Total Gross Loans	100.00%	100.00%
Multi-family | Non-Performing Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Non-Performing Loans, Amount	$ 0	$ 0
Non-Performing Loans, % of Total Gross Loans	0.00%	0.00%
Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Performing Loans, Amount	$ 1,717,535	$ 1,631,170
Commercial real estate | Performing Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Performing Loans, Amount	$ 1,713,161	$ 1,625,335
Performing Loans, % of Total Gross Loans	99.70%	99.60%
Commercial real estate | Non-Performing Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Non-Performing Loans, Amount	$ 4,374	$ 5,835
Non-Performing Loans, % of Total Gross Loans	0.30%	0.40%
Commercial & industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Performing Loans, Amount	$ 1,371,128	$ 1,268,695
Commercial & industrial | Performing Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Performing Loans, Amount	$ 1,370,658	$ 1,267,403
Performing Loans, % of Total Gross Loans	100.00%	99.90%
Commercial & industrial | Non-Performing Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Non-Performing Loans, Amount	$ 470	$ 1,292
Non-Performing Loans, % of Total Gross Loans	0.00%	0.10%
HELOC
Financing Receivable, Credit Quality Indicator [Line Items]
Performing Loans, Amount	$ 145,761	$ 142,178
HELOC | Performing Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Performing Loans, Amount	$ 144,697	$ 141,271
Performing Loans, % of Total Gross Loans	99.30%	99.40%
HELOC | Non-Performing Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Non-Performing Loans, Amount	$ 1,064	$ 907
Non-Performing Loans, % of Total Gross Loans	0.70%	0.60%
Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Performing Loans, Amount	$ 105,147	$ 129,883
Consumer | Performing Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Performing Loans, Amount	$ 105,147	$ 129,872
Performing Loans, % of Total Gross Loans	100.00%	100.00%
Consumer | Non-Performing Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Non-Performing Loans, Amount	$ 0	$ 11
Non-Performing Loans, % of Total Gross Loans	0.00%	0.00%