Allowance for Losses on Loans - Internally Assigned Grade (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Sep. 30, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	$ 13,434,649	$ 13,312,388
Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	$ 13,307,006	$ 13,208,853
Total grade as a % of total gross loans	99.05%	99.22%
Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	$ 8,982	$ 20,879
Total grade as a % of total gross loans	0.07%	0.16%
Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	$ 118,647	$ 82,656
Total grade as a % of total gross loans	0.88%	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	$ 5,582,244	$ 5,835,194
Single-family residential | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	5,557,959	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	24,285	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	2,204,283	2,038,052
Construction | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	2,200,930	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	3,353	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	547,731	540,741
Construction - custom | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	547,731	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	197,010	204,107
Land - acquisition & development | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	194,587	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	2,423	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	96,579	99,694
Land - consumer lot loans | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	95,901	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	678	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	1,467,231	1,422,674
Multi-family | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	1,462,588	1,418,837
Multi-family | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	4,643	0
Multi-family | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	0	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	1,717,535	1,631,170
Commercial real estate | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	1,694,801	1,602,634
Commercial real estate | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	2,718	2,754
Commercial real estate | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	20,016	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	1,371,128	1,268,695
Commercial & industrial | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	1,302,665	1,229,891
Commercial & industrial | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	1,621	18,125
Commercial & industrial | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	66,828	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	145,761	142,178
HELOC | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	144,697	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	1,064	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	105,147	129,883
Consumer | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable, gross	105,147	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	0	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