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Loans Receivable - Loans Individually Evaluated for Impairment (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2017	Mar. 31, 2018	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	$ 1,274,677	$ 1,274,677	$ 1,321,751
Loans individually evaluated for impairment by class loans
Loan Balance			6,007
Recorded Investment			5,196
Partial Charge-off			806
Average Investment in Impaired Loans			5,059
Interest Income Recognized			238
Allowance for loan losses (With an allowance recorded)	0	0	0
Impaired Loan, Total	4,069	4,069
Recorded Investment, Total	3,388	3,388
Partial Charge-offs, Total	699	699
Average Investment in Impaired Loans, Total	3,866	4,530
Interest Income Recognized, Total	23	65
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	8,103	8,103	8,366
Net deferred loan origination costs	1,213	1,213	1,266
Loans and Leases Receivable, Allowance	8,103	8,103	8,366	$ 8,179	$ 8,374	$ 8,374	$ 8,122	$ 8,127
Financing Receivable, Individually Evaluated for Impairment	3,385	3,385	5,214
Financing Receivable, Collectively Evaluated for Impairment	1,271,292	1,271,292	1,316,537
Loans, net	1,267,787	1,267,787	1,314,651
One-to-four family residential real estate loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	77,591	77,591	97,814
Loans individually evaluated for impairment by class loans
Loan Balance	3,322	3,322	5,049
Recorded Investment	2,677	2,677	4,248
Partial Charge-off	656	656	806
Average Investment in Impaired Loans	3,154	3,584	4,212
Interest Income Recognized	13	35	197
Allowance for loan losses (With an allowance recorded)	0	0	0
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	762	762	850
Loans and Leases Receivable, Allowance	762	762	850
Financing Receivable, Individually Evaluated for Impairment	2,724	2,724	4,265
Financing Receivable, Collectively Evaluated for Impairment	74,867	74,867	93,549
One-to-four family residential real estate loans - non-owner occupied loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	15,881	15,881	21,208
Loans individually evaluated for impairment by class loans
Loan Balance	86	86
Recorded Investment	46	46
Partial Charge-off	43	43
Average Investment in Impaired Loans	46	110
Interest Income Recognized	0	0
Multi-family mortgage loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	581,880	581,880	588,383
Loans individually evaluated for impairment by class loans
Loan Balance	661	661	958
Recorded Investment	665	665	948
Partial Charge-off	0	0	0
Average Investment in Impaired Loans	666	836	847
Interest Income Recognized	10	30	41
Allowance for loan losses (With an allowance recorded)	0	0	0
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	3,722	3,722	3,849
Loans and Leases Receivable, Allowance	3,722	3,722	3,849
Financing Receivable, Individually Evaluated for Impairment	661	661	949
Financing Receivable, Collectively Evaluated for Impairment	581,219	581,219	587,434
Multi Family Mortgage Loans without wholesale [Member] [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	276,810	276,810	291,358
Wholesale commercial lending [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	305,070	305,070	297,025
Nonresidential real estate loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	148,010	148,010	169,971
Loans individually evaluated for impairment by class loans
Allowance for loan losses (With an allowance recorded)	0	0	0
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	1,410	1,410	1,605
Loans and Leases Receivable, Allowance	1,410	1,410	1,605
Financing Receivable, Individually Evaluated for Impairment	0	0	0
Financing Receivable, Collectively Evaluated for Impairment	148,010	148,010	169,971
Construction Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	929	929	1,099
Land loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	201	201	259
Commercial loans - Secured [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	49,985	49,985	40,901
Commercial loans - other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	115,972	115,972	102,616
Consumer loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	1,416	1,416	1,597
Loans individually evaluated for impairment by class loans
Allowance for loan losses (With an allowance recorded)	0	0	0
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	16	16	18
Loans and Leases Receivable, Allowance	16	16	18
Financing Receivable, Individually Evaluated for Impairment	0	0	0
Financing Receivable, Collectively Evaluated for Impairment	1,416	1,416	1,597
Construction And Land Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	1,130	1,130	1,358
Loans individually evaluated for impairment by class loans
Allowance for loan losses (With an allowance recorded)	0	0	0
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	27	27	32
Loans and Leases Receivable, Allowance	27	27	32
Financing Receivable, Individually Evaluated for Impairment	0	0	0
Financing Receivable, Collectively Evaluated for Impairment	1,130	1,130	1,358
Commercial Loan [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	167,547	167,547	152,552
Loans individually evaluated for impairment by class loans
Allowance for loan losses (With an allowance recorded)	0	0	0
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	1,482	1,482	1,357
Loans and Leases Receivable, Allowance	1,482	1,482	1,357
Financing Receivable, Individually Evaluated for Impairment	0	0	0
Financing Receivable, Collectively Evaluated for Impairment	167,547	167,547	152,552
Finance Leases Financing Receivable [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	297,103	297,103	310,076
Loans individually evaluated for impairment by class loans
Allowance for loan losses (With an allowance recorded)	0	0	0
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	684	684	655
Loans and Leases Receivable, Allowance	684	684	655
Financing Receivable, Individually Evaluated for Impairment	0	0	0
Financing Receivable, Collectively Evaluated for Impairment	297,103	297,103	310,076
One to Four Family Residential Real Estate Loans-Owner occupied [Member] [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	$ 61,710	$ 61,710	$ 76,606

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