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, 2019	Dec. 31, 2018	Sep. 30, 2017
Financing Receivable, Impaired [Line Items]
Average recorded Investment	$ 25,590	$ 21,240	$ 19,357	$ 27,054
Interest Income Recognized	230	1,203	891	1,079
Real estate loans \| One-to-four family
Financing Receivable, Impaired [Line Items]
Average recorded Investment	14,015	8,811	10,224	15,027
Interest Income Recognized	110	434	445	469
Real estate loans \| Multifamily and commercial
Financing Receivable, Impaired [Line Items]
Average recorded Investment	4,087	2,639	2,712	4,328
Interest Income Recognized	39	147	155	279
Real estate loans \| Construction
Financing Receivable, Impaired [Line Items]
Average recorded Investment		850	0
Interest Income Recognized		0	0
Commercial business loans
Financing Receivable, Impaired [Line Items]
Average recorded Investment	3,870	6,378	3,060	3,796
Interest Income Recognized	46	479	118	195
Consumer loans \| Home equity loans and advances
Financing Receivable, Impaired [Line Items]
Average recorded Investment	3,618	2,562	3,361	3,903
Interest Income Recognized	$ 35	$ 143	$ 173	$ 136

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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=118936363&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=118936363&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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