Loans Receivable and Allowance for Loan Losses - Troubled Debt Restructuring (Details) $ in Thousands	12 Months Ended
Dec. 31, 2018 USD ($) loan	Dec. 31, 2017 USD ($) loan	Sep. 30, 2017 USD ($) loan	Sep. 30, 2016 USD ($) loan
Financing Receivable, Modifications [Line Items]
No. of Loans \| loan	7	0	7	4
Pre-modification recorded investment	$ 1,454	$ 0	$ 4,778	$ 358
Post-modification recorded investment	$ 1,454	$ 0	$ 4,778	$ 358
Real estate loans \| One-to-four family
Financing Receivable, Modifications [Line Items]
No. of Loans \| loan	5	0	3	0
Pre-modification recorded investment	$ 801	$ 0	$ 548	$ 0
Post-modification recorded investment	$ 801	$ 0	$ 548	$ 0
Real estate loans \| Multifamily and commercial
Financing Receivable, Modifications [Line Items]
No. of Loans \| loan	1	0	1	0
Pre-modification recorded investment	$ 65	$ 0	$ 3,964	$ 0
Post-modification recorded investment	$ 65	$ 0	$ 3,964	$ 0
Commercial business loans \| Commercial business loans
Financing Receivable, Modifications [Line Items]
No. of Loans \| loan	0	0	1	3
Pre-modification recorded investment	$ 0	$ 0	$ 18	$ 255
Post-modification recorded investment	$ 0	$ 0	$ 18	$ 255
Consumer loans \| Home equity loans and advances
Financing Receivable, Modifications [Line Items]
No. of Loans \| loan	1	0	2	1
Pre-modification recorded investment	$ 588	$ 0	$ 248	$ 103
Post-modification recorded investment	$ 588	$ 0	$ 248	$ 103

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Number of financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953676-111524

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