Allowance for Losses on Loans - Credit Risk Profile Based on Payment Activity (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Sep. 30, 2017
Financing Receivable, Recorded Investment [Line Items]
Performing Loans, Amount	$ 12,737,292	$ 12,201,387
Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Performing Loans, Amount	$ 12,676,708	$ 12,151,807
Performing Loans, % of Total Gross Loans	99.50%	99.60%
Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Performing Loans, Amount	$ 71,859	$ 70,238
Non-Performing Loans, Amount	$ 60,584	$ 49,580
Non-Performing Loans, % of Total Gross Loans	0.50%	0.40%
Single-family residential
Financing Receivable, Recorded Investment [Line Items]
Performing Loans, Amount	$ 5,745,598	$ 5,711,004
Single-family residential | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Performing Loans, Amount	$ 5,719,479	$ 5,683,074
Performing Loans, % of Total Gross Loans	99.50%	99.50%
Single-family residential | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Non-Performing Loans, Amount	$ 26,119	$ 27,930
Non-Performing Loans, % of Total Gross Loans	0.50%	0.50%
Construction
Financing Receivable, Recorded Investment [Line Items]
Performing Loans, Amount	$ 1,885,034	$ 1,597,996
Construction | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Performing Loans, Amount	$ 1,883,193	$ 1,597,996
Performing Loans, % of Total Gross Loans	99.90%	100.00%
Construction | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Non-Performing Loans, Amount	$ 1,841	$ 0
Non-Performing Loans, % of Total Gross Loans	0.10%	0.00%
Construction - custom
Financing Receivable, Recorded Investment [Line Items]
Performing Loans, Amount	$ 612,688	$ 602,631
Construction - custom | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Performing Loans, Amount	$ 612,688	$ 602,540
Performing Loans, % of Total Gross Loans	100.00%	99.90%
Construction - custom | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Non-Performing Loans, Amount	$ 0	$ 91
Non-Performing Loans, % of Total Gross Loans	0.00%	0.10%
Land - acquisition & development
Financing Receivable, Recorded Investment [Line Items]
Performing Loans, Amount	$ 150,936	$ 124,308
Land - acquisition & development | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Performing Loans, Amount	$ 149,179	$ 124,012
Performing Loans, % of Total Gross Loans	98.80%	99.80%
Land - acquisition & development | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Non-Performing Loans, Amount	$ 1,757	$ 296
Non-Performing Loans, % of Total Gross Loans	1.20%	0.20%
Land - consumer lot loans
Financing Receivable, Recorded Investment [Line Items]
Performing Loans, Amount	$ 103,118	$ 104,405
Land - consumer lot loans | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Performing Loans, Amount	$ 102,476	$ 103,800
Performing Loans, % of Total Gross Loans	99.40%	99.40%
Land - consumer lot loans | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Non-Performing Loans, Amount	$ 642	$ 605
Non-Performing Loans, % of Total Gross Loans	0.60%	0.60%
Multi-family
Financing Receivable, Recorded Investment [Line Items]
Performing Loans, Amount	$ 1,346,534	$ 1,303,148
Multi-family | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Performing Loans, Amount	$ 1,346,534	$ 1,303,009
Performing Loans, % of Total Gross Loans	100.00%	99.90%
Multi-family | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Non-Performing Loans, Amount	$ 0	$ 139
Non-Performing Loans, % of Total Gross Loans	0.00%	0.10%
Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Performing Loans, Amount	$ 1,435,418	$ 1,434,610
Commercial real estate | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Performing Loans, Amount	$ 1,425,734	$ 1,422,795
Performing Loans, % of Total Gross Loans	99.30%	99.20%
Commercial real estate | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Non-Performing Loans, Amount	$ 9,684	$ 11,815
Non-Performing Loans, % of Total Gross Loans	0.70%	0.80%
Commercial & industrial
Financing Receivable, Recorded Investment [Line Items]
Performing Loans, Amount	$ 1,133,075	$ 1,093,360
Commercial & industrial | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Performing Loans, Amount	$ 1,113,199	$ 1,085,278
Performing Loans, % of Total Gross Loans	98.20%	99.30%
Commercial & industrial | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Non-Performing Loans, Amount	$ 19,876	$ 8,082
Non-Performing Loans, % of Total Gross Loans	1.80%	0.70%
HELOC
Financing Receivable, Recorded Investment [Line Items]
Performing Loans, Amount	$ 136,766	$ 144,850
HELOC | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Performing Loans, Amount	$ 136,129	$ 144,319
Performing Loans, % of Total Gross Loans	99.50%	99.60%
HELOC | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Non-Performing Loans, Amount	$ 637	$ 531
Non-Performing Loans, % of Total Gross Loans	0.50%	0.40%
Consumer
Financing Receivable, Recorded Investment [Line Items]
Performing Loans, Amount	$ 188,125	$ 85,075
Consumer | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Performing Loans, Amount	$ 188,097	$ 84,984
Performing Loans, % of Total Gross Loans	100.00%	99.90%
Consumer | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Non-Performing Loans, Amount	$ 28	$ 91
Non-Performing Loans, % of Total Gross Loans	0.00%	0.10%