Allowance for Losses on Loans - Internally Assigned Grade (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Sep. 30, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	$ 13,404,413	$ 13,312,388
Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	$ 13,308,799	$ 13,208,853
Total grade as a % of total gross loans	99.29%	99.22%
Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	$ 4,140	$ 20,879
Total grade as a % of total gross loans	0.03%	0.16%
Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	$ 91,460	$ 82,656
Total grade as a % of total gross loans	0.68%	0.62%
Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	$ 14	$ 0
Total grade as a % of total gross loans	0.00%	0.00%
Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	$ 0	$ 0
Total grade as a % of total gross loans	0.00%	0.00%
Single-family residential
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	$ 5,702,071	$ 5,835,194
Single-family residential | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	5,676,712	5,808,444
Single-family residential | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	0	0
Single-family residential | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	25,359	26,750
Single-family residential | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	0	0
Single-family residential | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	0	0
Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	2,174,313	2,038,052
Construction | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	2,174,313	2,038,052
Construction | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	0	0
Construction | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	0	0
Construction | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	0	0
Construction | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	0	0
Construction - custom
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	538,234	540,741
Construction - custom | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	538,234	540,741
Construction - custom | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	0	0
Construction - custom | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	0	0
Construction - custom | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	0	0
Construction - custom | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	0	0
Land - acquisition & development
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	203,043	204,107
Land - acquisition & development | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	200,533	200,283
Land - acquisition & development | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	0	0
Land - acquisition & development | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	2,510	3,824
Land - acquisition & development | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	0	0
Land - acquisition & development | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	0	0
Land - consumer lot loans
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	97,097	99,694
Land - consumer lot loans | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	96,627	98,828
Land - consumer lot loans | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	0	0
Land - consumer lot loans | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	470	866
Land - consumer lot loans | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	0	0
Land - consumer lot loans | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	0	0
Multi-family
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	1,436,715	1,422,674
Multi-family | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	1,432,895	1,418,837
Multi-family | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	0	0
Multi-family | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	3,820	3,837
Multi-family | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	0	0
Multi-family | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	0	0
Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	1,643,099	1,631,170
Commercial real estate | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	1,619,197	1,602,634
Commercial real estate | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	3,426	2,754
Commercial real estate | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	20,476	25,782
Commercial real estate | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	0	0
Commercial real estate | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	0	0
Commercial & industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	1,352,738	1,268,695
Commercial & industrial | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	1,313,816	1,229,891
Commercial & industrial | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	714	18,125
Commercial & industrial | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	38,194	20,679
Commercial & industrial | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	14	0
Commercial & industrial | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	0	0
HELOC
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	141,274	142,178
HELOC | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	140,644	141,271
HELOC | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	0	0
HELOC | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	630	907
HELOC | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	0	0
HELOC | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	0	0
Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	115,829	129,883
Consumer | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	115,828	129,872
Consumer | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	0	0
Consumer | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	1	11
Consumer | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	0	0
Consumer | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	$ 0	$ 0