Loans Receivable and Allowance for Loan Losses - Interest Income on Impaired Loans (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Dec. 31, 2017	Dec. 31, 2018	Sep. 30, 2017	Sep. 30, 2016
Financing Receivable, Impaired [Line Items]
Average recorded Investment	$ 25,590	$ 19,357	$ 27,054	$ 35,928
Interest Income Recognized	230	891	1,079	889
Real estate loans \| One-to-four family
Financing Receivable, Impaired [Line Items]
Average recorded Investment	14,015	10,224	15,027	18,119
Interest Income Recognized	110	445	469	565
Real estate loans \| Multifamily and commercial
Financing Receivable, Impaired [Line Items]
Average recorded Investment	4,087	2,712	4,328	9,344
Interest Income Recognized	39	155	279	57
Real estate loans \| Construction
Financing Receivable, Impaired [Line Items]
Average recorded Investment			0	505
Interest Income Recognized			0	0
Commercial business loans
Financing Receivable, Impaired [Line Items]
Average recorded Investment	3,870	3,060	3,796	4,514
Interest Income Recognized	46	118	195	110
Consumer loans \| Home equity loans and advances
Financing Receivable, Impaired [Line Items]
Average recorded Investment	3,618	3,361	3,903	3,446
Interest Income Recognized	$ 35	$ 173	$ 136	$ 157

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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://fasb.org/us-gaap/role/ref/legacyRef
-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://fasb.org/us-gaap/role/ref/legacyRef
-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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