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Loans Receivable - Loans Individually Evaluated for Impairment (Details) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended | 12 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Jun. 30, 2018 | Jun. 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 | $ 1,294,826 | $ 1,294,826 | $ 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 | 4,859 | 4,859 |  |  |  |  |  |
Recorded Investment, Total | 4,167 | 4,167 |  |  |  |  |  |
Partial Charge-offs, Total | 692 | 692 |  |  |  |  |  |
Average Investment in Impaired Loans, Total | 4,470 | 4,858 |  |  |  |  |  |
Interest Income Recognized, Total | 21 | 44 |  |  |  |  |  |
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment | 8,179 | 8,179 | 8,366 |  |  |  |  |
Net deferred loan origination costs | 1,176 | 1,176 | 1,266 |  |  |  |  |
Loans and Leases Receivable, Allowance | 8,179 | 8,179 | 8,366 | $ 8,341 | $ 8,122 | $ 8,122 | $ 7,971 | $ 8,127
Financing Receivable, Individually Evaluated for Impairment | 4,182 | 4,182 | 5,214 |  |  |  |  |
Financing Receivable, Collectively Evaluated for Impairment | 1,290,644 | 1,290,644 | 1,316,537 |  |  |  |  |
Loans, net | 1,287,823 | 1,287,823 | 1,314,651 |  |  |  |  |
One-to-four family residential real estate loans [Member] |  |  |  |  |  |  |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |  |  |  |  |  |  |
Total loans | 84,048 | 84,048 | 97,814 |  |  |  |  |
Loans individually evaluated for impairment by class loans |  |  |  |  |  |  |  |
Loan Balance | 4,105 | 4,105 | 5,049 |  |  |  |  |
Recorded Investment | 3,455 | 3,455 | 4,248 |  |  |  |  |
Partial Charge-off | 650 | 650 | 806 |  |  |  |  |
Average Investment in Impaired Loans | 3,558 | 3,812 | 4,212 |  |  |  |  |
Interest Income Recognized | 12 | 24 | 197 |  |  |  |  |
Allowance for loan losses (With an allowance recorded) | 0 | 0 | 0 |  |  |  |  |
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment | 802 | 802 | 850 |  |  |  |  |
Loans and Leases Receivable, Allowance | 802 | 802 | 850 |  |  |  |  |
Financing Receivable, Individually Evaluated for Impairment | 3,514 | 3,514 | 4,265 |  |  |  |  |
Financing Receivable, Collectively Evaluated for Impairment | 80,534 | 80,534 | 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 | 17,066 | 17,066 | 21,208 |  |  |  |  |
Loans individually evaluated for impairment by class loans |  |  |  |  |  |  |  |
Loan Balance | 86 | 86 |  |  |  |  |  |
Recorded Investment | 46 | 46 |  |  |  |  |  |
Partial Charge-off | 42 | 42 |  |  |  |  |  |
Average Investment in Impaired Loans | 35 | 137 |  |  |  |  |  |
Interest Income Recognized | 0 | 0 |  |  |  |  |  |
Multi-family mortgage loans [Member] |  |  |  |  |  |  |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |  |  |  |  |  |  |
Total loans | 571,886 | 571,886 | 588,383 |  |  |  |  |
Loans individually evaluated for impairment by class loans |  |  |  |  |  |  |  |
Loan Balance | 668 | 668 | 958 |  |  |  |  |
Recorded Investment | 666 | 666 | 948 |  |  |  |  |
Partial Charge-off | 0 | 0 | 0 |  |  |  |  |
Average Investment in Impaired Loans | 877 | 909 | 847 |  |  |  |  |
Interest Income Recognized | 9 | 20 | 41 |  |  |  |  |
Allowance for loan losses (With an allowance recorded) | 0 | 0 | 0 |  |  |  |  |
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment | 3,690 | 3,690 | 3,849 |  |  |  |  |
Loans and Leases Receivable, Allowance | 3,690 | 3,690 | 3,849 |  |  |  |  |
Financing Receivable, Individually Evaluated for Impairment | 668 | 668 | 949 |  |  |  |  |
Financing Receivable, Collectively Evaluated for Impairment | 571,218 | 571,218 | 587,434 |  |  |  |  |
Multi Family Mortgage Loans without wholesale [Member] [Member] |  |  |  |  |  |  |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |  |  |  |  |  |  |
Total loans | 271,224 | 271,224 | 291,358 |  |  |  |  |
Wholesale commercial lending [Member] |  |  |  |  |  |  |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |  |  |  |  |  |  |
Total loans | 300,662 | 300,662 | 297,025 |  |  |  |  |
Nonresidential real estate loans [Member] |  |  |  |  |  |  |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |  |  |  |  |  |  |
Total loans | 155,627 | 155,627 | 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 | 1,452 | 1,452 | 1,605 |  |  |  |  |
Loans and Leases Receivable, Allowance | 1,452 | 1,452 | 1,605 |  |  |  |  |
Financing Receivable, Individually Evaluated for Impairment | 0 | 0 | 0 |  |  |  |  |
Financing Receivable, Collectively Evaluated for Impairment | 155,627 | 155,627 | 169,971 |  |  |  |  |
Construction Loans [Member] |  |  |  |  |  |  |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |  |  |  |  |  |  |
Total loans | 1,086 | 1,086 | 1,099 |  |  |  |  |
Land loans [Member] |  |  |  |  |  |  |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |  |  |  |  |  |  |
Total loans | 230 | 230 | 259 |  |  |  |  |
Commercial loans - Secured [Member] |  |  |  |  |  |  |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |  |  |  |  |  |  |
Total loans | 46,303 | 46,303 | 40,901 |  |  |  |  |
Commercial loans - other [Member] |  |  |  |  |  |  |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |  |  |  |  |  |  |
Total loans | 118,809 | 118,809 | 102,616 |  |  |  |  |
Consumer loans [Member] |  |  |  |  |  |  |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |  |  |  |  |  |  |
Total loans | 1,469 | 1,469 | 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 | 13 | 13 | 18 |  |  |  |  |
Loans and Leases Receivable, Allowance | 13 | 13 | 18 |  |  |  |  |
Financing Receivable, Individually Evaluated for Impairment | 0 | 0 | 0 |  |  |  |  |
Financing Receivable, Collectively Evaluated for Impairment | 1,469 | 1,469 | 1,597 |  |  |  |  |
Construction And Land Loans [Member] |  |  |  |  |  |  |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |  |  |  |  |  |  |
Total loans | 1,316 | 1,316 | 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 | 32 | 32 | 32 |  |  |  |  |
Loans and Leases Receivable, Allowance | 32 | 32 | 32 |  |  |  |  |
Financing Receivable, Individually Evaluated for Impairment | 0 | 0 | 0 |  |  |  |  |
Financing Receivable, Collectively Evaluated for Impairment | 1,316 | 1,316 | 1,358 |  |  |  |  |
Commercial Loan [Member] |  |  |  |  |  |  |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |  |  |  |  |  |  |
Total loans | 163,925 | 163,925 | 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 | 1,444 | 1,444 | 1,357 |  |  |  |  |
Loans and Leases Receivable, Allowance | 1,444 | 1,444 | 1,357 |  |  |  |  |
Financing Receivable, Individually Evaluated for Impairment | 0 | 0 | 0 |  |  |  |  |
Financing Receivable, Collectively Evaluated for Impairment | 163,925 | 163,925 | 152,552 |  |  |  |  |
Finance Leases Financing Receivable [Member] |  |  |  |  |  |  |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |  |  |  |  |  |  |
Total loans | 316,555 | 316,555 | 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 | 746 | 746 | 655 |  |  |  |  |
Loans and Leases Receivable, Allowance | 746 | 746 | 655 |  |  |  |  |
Financing Receivable, Individually Evaluated for Impairment | 0 | 0 | 0 |  |  |  |  |
Financing Receivable, Collectively Evaluated for Impairment | 316,555 | 316,555 | 310,076 |  |  |  |  |
One to Four Family Residential Real Estate Loans-Owner occupied [Member] [Member] |  |  |  |  |  |  |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |  |  |  |  |  |  |
Total loans | $ 66,982 | $ 66,982 | $ 76,606 |  |  |  |  |

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