Loans Receivable and Allowance for Loan Losses - Interest Income on Impaired Loans (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Financing Receivable, Impaired [Line Items]
Average recorded Investment	$ 21,044	$ 27,805	$ 22,668	$ 30,513
Interest Income Recognized	190	217	420	488
Real estate loans \| One-to-four family
Financing Receivable, Impaired [Line Items]
Average recorded Investment	11,235	16,061	11,572	16,747
Interest Income Recognized	102	142	212	260
Real estate loans \| Multifamily and commercial
Financing Receivable, Impaired [Line Items]
Average recorded Investment	3,136	3,825	4,205	5,665
Interest Income Recognized	26	14	65	84
Real estate loans \| Construction
Financing Receivable, Impaired [Line Items]
Average recorded Investment			0	168
Interest Income Recognized			0	0
Commercial business loans
Financing Receivable, Impaired [Line Items]
Average recorded Investment	3,836	3,797	4,000	4,036
Interest Income Recognized	26	26	72	75
Consumer loans
Financing Receivable, Impaired [Line Items]
Average recorded Investment
Interest Income Recognized
Consumer loans \| Home equity loans and advances
Financing Receivable, Impaired [Line Items]
Average recorded Investment	2,837	4,122	2,891	3,897
Interest Income Recognized	$ 36	$ 35	$ 71	$ 69

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- Definition

Average amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5212-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5258-111524

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- Definition

Amount of interest income recognized that the financing receivables (with related allowance for credit losses and without a related allowance for credit losses) were impaired.

+ References

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