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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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