Loans Receivable, Part IV (Details) - USD ($) | 3 Months Ended | 6 Months Ended | 12 Months Ended | |
Mar. 31, 2018 | Jun. 30, 2018 | Dec. 31, 2015 | Dec. 31, 2017 |
Loans and Leases Receivable [Abstract] | | | | |
At December 31, 2016 | | $ 1,342,136,000 | | $ 1,291,814,000 |
Loans requiring an officers' loans committee review and approval, minimum | | 500,000 | | |
Minimum amount of potential loan exposure to be reviewed by credit management personnel | | $ 1,000,000 | | |
Maximum period of time loan is reviewed if renewed | | 1 year | | |
Period of time sales of real estate owned fair value is based | | 2 years | | |
Impaired Loans Receivable, Recorded Investment [Abstract] | | | | |
Total Impaired, with Reserve, Recorded Investment | | $ 543,000 | | 1,016,000 |
Total Impaired, with no Reserve, Recorded Investment | | 8,618,000 | | 8,160,000 |
Total Impaired, Recorded Investment | | 9,161,000 | | 9,176,000 |
Impaired Loans Receivable, Unpaid Principal Balance [Abstract] | | | | |
Total Impaired with Reserve, Unpaid Principal Balance | | 952,000 | | 1,425,000 |
Total Impaired with no Reserve, Unpaid Principal Balance | | 10,495,000 | | 10,106,000 |
Total Impaired, Unpaid Principal Balance, Total | | 11,447,000 | | 11,531,000 |
Impaired Loans Receivable, Reserve [Abstract] | | | | |
Total Impaired with Reserve, Reserve | | 111,000 | | 155,000 |
Total Impaired with no Reserve, Reserve | | 0 | | 0 |
Impaired Loans, Reserve | | 111,000 | | 155,000 |
Impaired Loans Receivable, Cumulative Charge Offs [Abstract] | | | | |
Total Impaired with Reserve, Cumulative Charge-offs | | 409,000 | | 409,000 |
Total Impaired with no Reserve, Cumulative Charge-offs | | 1,877,000 | | 1,946,000 |
Total Impaired, Cumulative Charge-offs | | 2,286,000 | | 2,355,000 |
Impaired Loans Receivable, Average Recorded Investment [Abstract] | | | | |
Total Impaired with Reserve, Average Recorded Investment | | 554,000 | $ 1,039,000 | |
Total Impaired with no Reserve, Average Recorded Investment | | 8,708,000 | 8,451,000 | |
Total Impaired, Average Recorded Investment, Total | | 9,262,000 | 9,490,000 | |
Impaired Loans Receivable, Interest Paid YTD [Abstract] | | | | |
Total Impaired with Reserve, Interest Paid YTD | | 18,000 | 58,000 | |
Total Impaired with no Reserve, Interest Paid YTD | | 228,000 | 477,000 | |
Total Impaired, Interest Paid YTD | | 246,000 | 535,000 | |
Charge-offs recorded to reduce the unpaid principal balance | | 1,900,000 | | |
Total impaired loans including troubled debt restructurings | | 4,384,000 | | 5,082,000 |
Substandard [Member] | | | | |
Loans and Leases Receivable [Abstract] | | | | |
At December 31, 2016 | | 10,191,000 | | 11,563,000 |
Watch [Member] | | | | |
Loans and Leases Receivable [Abstract] | | | | |
At December 31, 2016 | | 8,347,000 | | 6,706,000 |
Pass [Member] | | | | |
Loans and Leases Receivable [Abstract] | | | | |
At December 31, 2016 | | 1,323,598,000 | | 1,273,545,000 |
Residential Real Estate [Member] | One-to-Four Family [Member] | | | | |
Loans and Leases Receivable [Abstract] | | | | |
At December 31, 2016 | | 468,371,000 | | 439,597,000 |
Impaired Loans Receivable, Recorded Investment [Abstract] | | | | |
Total Impaired, with Reserve, Recorded Investment | | 440,000 | | 903,000 |
Total Impaired, with no Reserve, Recorded Investment | | 7,229,000 | | 6,515,000 |
Total Impaired, Recorded Investment | | 7,669,000 | | 7,418,000 |
Impaired Loans Receivable, Unpaid Principal Balance [Abstract] | | | | |
Total Impaired with Reserve, Unpaid Principal Balance | | 440,000 | | 903,000 |
Total Impaired with no Reserve, Unpaid Principal Balance | | 8,260,000 | | 7,604,000 |
Total Impaired, Unpaid Principal Balance, Total | | 8,700,000 | | 8,507,000 |
Impaired Loans Receivable, Reserve [Abstract] | | | | |
Total Impaired with Reserve, Reserve | | 51,000 | | 77,000 |
Total Impaired with no Reserve, Reserve | | 0 | | 0 |
Impaired Loans, Reserve | | 51,000 | | 77,000 |
Impaired Loans Receivable, Cumulative Charge Offs [Abstract] | | | | |
Total Impaired with Reserve, Cumulative Charge-offs | | 0 | | 0 |
Total Impaired with no Reserve, Cumulative Charge-offs | | 1,031,000 | | 1,089,000 |
Total Impaired, Cumulative Charge-offs | | 1,031,000 | | 1,089,000 |
Impaired Loans Receivable, Average Recorded Investment [Abstract] | | | | |
Total Impaired with Reserve, Average Recorded Investment | | 444,000 | 913,000 | |
Total Impaired with no Reserve, Average Recorded Investment | | 7,307,000 | 6,796,000 | |
Total Impaired, Average Recorded Investment, Total | | 7,751,000 | 7,709,000 | |
Impaired Loans Receivable, Interest Paid YTD [Abstract] | | | | |
Total Impaired with Reserve, Interest Paid YTD | | 15,000 | 52,000 | |
Total Impaired with no Reserve, Interest Paid YTD | | 193,000 | 359,000 | |
Total Impaired, Interest Paid YTD | | 208,000 | 411,000 | |
Total impaired loans including troubled debt restructurings | | 3,674,000 | | 3,896,000 |
Residential Real Estate [Member] | One-to-Four Family [Member] | Substandard [Member] | | | | |
Loans and Leases Receivable [Abstract] | | | | |
At December 31, 2016 | | 7,829,000 | | 7,581,000 |
Residential Real Estate [Member] | One-to-Four Family [Member] | Watch [Member] | | | | |
Loans and Leases Receivable [Abstract] | | | | |
At December 31, 2016 | | 6,018,000 | | 4,939,000 |
Residential Real Estate [Member] | One-to-Four Family [Member] | Pass [Member] | | | | |
Loans and Leases Receivable [Abstract] | | | | |
At December 31, 2016 | | 454,524,000 | | 427,077,000 |
Residential Real Estate [Member] | Multi Family [Member] | | | | |
Loans and Leases Receivable [Abstract] | | | | |
At December 31, 2016 | | 592,620,000 | | 578,440,000 |
Impaired Loans Receivable, Recorded Investment [Abstract] | | | | |
Total Impaired, with Reserve, Recorded Investment | | 0 | | 0 |
Total Impaired, with no Reserve, Recorded Investment | | 781,000 | | 1,007,000 |
Total Impaired, Recorded Investment | | 781,000 | | 1,007,000 |
Impaired Loans Receivable, Unpaid Principal Balance [Abstract] | | | | |
Total Impaired with Reserve, Unpaid Principal Balance | | 0 | | 0 |
Total Impaired with no Reserve, Unpaid Principal Balance | | 1,627,000 | | 1,864,000 |
Total Impaired, Unpaid Principal Balance, Total | | 1,627,000 | | 1,864,000 |
Impaired Loans Receivable, Reserve [Abstract] | | | | |
Total Impaired with Reserve, Reserve | | 0 | | 0 |
Total Impaired with no Reserve, Reserve | | 0 | | 0 |
Impaired Loans, Reserve | | 0 | | 0 |
Impaired Loans Receivable, Cumulative Charge Offs [Abstract] | | | | |
Total Impaired with Reserve, Cumulative Charge-offs | | 0 | | 0 |
Total Impaired with no Reserve, Cumulative Charge-offs | | 846,000 | | 857,000 |
Total Impaired, Cumulative Charge-offs | | 846,000 | | 857,000 |
Impaired Loans Receivable, Average Recorded Investment [Abstract] | | | | |
Total Impaired with Reserve, Average Recorded Investment | | 0 | 0 | |
Total Impaired with no Reserve, Average Recorded Investment | | 791,000 | 1,005,000 | |
Total Impaired, Average Recorded Investment, Total | | 791,000 | 1,005,000 | |
Impaired Loans Receivable, Interest Paid YTD [Abstract] | | | | |
Total Impaired with Reserve, Interest Paid YTD | | 0 | 0 | |
Total Impaired with no Reserve, Interest Paid YTD | | 26,000 | 94,000 | |
Total Impaired, Interest Paid YTD | | 26,000 | 94,000 | |
Total impaired loans including troubled debt restructurings | | 397,000 | | 815,000 |
Residential Real Estate [Member] | Multi Family [Member] | Substandard [Member] | | | | |
Loans and Leases Receivable [Abstract] | | | | |
At December 31, 2016 | | 781,000 | | 1,135,000 |
Residential Real Estate [Member] | Multi Family [Member] | Watch [Member] | | | | |
Loans and Leases Receivable [Abstract] | | | | |
At December 31, 2016 | | 544,000 | | 330,000 |
Residential Real Estate [Member] | Multi Family [Member] | Pass [Member] | | | | |
Loans and Leases Receivable [Abstract] | | | | |
At December 31, 2016 | | 591,295,000 | | 576,975,000 |
Residential Real Estate [Member] | Home Equity [Member] | | | | |
Loans and Leases Receivable [Abstract] | | | | |
At December 31, 2016 | | 20,247,000 | | 21,124,000 |
Impaired Loans Receivable, Recorded Investment [Abstract] | | | | |
Total Impaired, with Reserve, Recorded Investment | | 78,000 | | 79,000 |
Total Impaired, with no Reserve, Recorded Investment | | 145,000 | | 106,000 |
Total Impaired, Recorded Investment | | 223,000 | | 185,000 |
Impaired Loans Receivable, Unpaid Principal Balance [Abstract] | | | | |
Total Impaired with Reserve, Unpaid Principal Balance | | 78,000 | | 79,000 |
Total Impaired with no Reserve, Unpaid Principal Balance | | 145,000 | | 106,000 |
Total Impaired, Unpaid Principal Balance, Total | | 223,000 | | 185,000 |
Impaired Loans Receivable, Reserve [Abstract] | | | | |
Total Impaired with Reserve, Reserve | | 34,000 | | 44,000 |
Total Impaired with no Reserve, Reserve | | 0 | | 0 |
Impaired Loans, Reserve | | 34,000 | | 44,000 |
Impaired Loans Receivable, Cumulative Charge Offs [Abstract] | | | | |
Total Impaired with Reserve, Cumulative Charge-offs | | 0 | | 0 |
Total Impaired with no Reserve, Cumulative Charge-offs | | 0 | | 0 |
Total Impaired, Cumulative Charge-offs | | 0 | | 0 |
Impaired Loans Receivable, Average Recorded Investment [Abstract] | | | | |
Total Impaired with Reserve, Average Recorded Investment | | 80,000 | 83,000 | |
Total Impaired with no Reserve, Average Recorded Investment | | 146,000 | 111,000 | |
Total Impaired, Average Recorded Investment, Total | | 226,000 | 194,000 | |
Impaired Loans Receivable, Interest Paid YTD [Abstract] | | | | |
Total Impaired with Reserve, Interest Paid YTD | | 3,000 | 6,000 | |
Total Impaired with no Reserve, Interest Paid YTD | | 2,000 | 5,000 | |
Total Impaired, Interest Paid YTD | | 5,000 | 11,000 | |
Total impaired loans including troubled debt restructurings | | | | 47,000 |
Residential Real Estate [Member] | Home Equity [Member] | Substandard [Member] | | | | |
Loans and Leases Receivable [Abstract] | | | | |
At December 31, 2016 | | 256,000 | | 138,000 |
Residential Real Estate [Member] | Home Equity [Member] | Watch [Member] | | | | |
Loans and Leases Receivable [Abstract] | | | | |
At December 31, 2016 | | 378,000 | | 401,000 |
Residential Real Estate [Member] | Home Equity [Member] | Pass [Member] | | | | |
Loans and Leases Receivable [Abstract] | | | | |
At December 31, 2016 | | 19,613,000 | | 20,585,000 |
Construction and Land [Member] | | | | |
Loans and Leases Receivable [Abstract] | | | | |
At December 31, 2016 | | 10,122,000 | | 19,859,000 |
Impaired Loans Receivable, Recorded Investment [Abstract] | | | | |
Total Impaired, with Reserve, Recorded Investment | | 0 | | 0 |
Total Impaired, with no Reserve, Recorded Investment | | 0 | | 0 |
Total Impaired, Recorded Investment | | 0 | | 0 |
Impaired Loans Receivable, Unpaid Principal Balance [Abstract] | | | | |
Total Impaired with Reserve, Unpaid Principal Balance | | 0 | | 0 |
Total Impaired with no Reserve, Unpaid Principal Balance | | 0 | | 0 |
Total Impaired, Unpaid Principal Balance, Total | | 0 | | 0 |
Impaired Loans Receivable, Reserve [Abstract] | | | | |
Total Impaired with Reserve, Reserve | | 0 | | 0 |
Total Impaired with no Reserve, Reserve | | 0 | | 0 |
Impaired Loans, Reserve | | 0 | | 0 |
Impaired Loans Receivable, Cumulative Charge Offs [Abstract] | | | | |
Total Impaired with Reserve, Cumulative Charge-offs | | 0 | | 0 |
Total Impaired with no Reserve, Cumulative Charge-offs | | 0 | | 0 |
Total Impaired, Cumulative Charge-offs | | 0 | | 0 |
Impaired Loans Receivable, Average Recorded Investment [Abstract] | | | | |
Total Impaired with Reserve, Average Recorded Investment | $ 0 | | 0 | |
Total Impaired with no Reserve, Average Recorded Investment | | 0 | 0 | |
Total Impaired, Average Recorded Investment, Total | | 0 | 0 | |
Impaired Loans Receivable, Interest Paid YTD [Abstract] | | | | |
Total Impaired with Reserve, Interest Paid YTD | | 0 | 0 | |
Total Impaired with no Reserve, Interest Paid YTD | | 0 | 0 | |
Total Impaired, Interest Paid YTD | | 0 | 0 | |
Construction and Land [Member] | Substandard [Member] | | | | |
Loans and Leases Receivable [Abstract] | | | | |
At December 31, 2016 | | 0 | | 0 |
Construction and Land [Member] | Watch [Member] | | | | |
Loans and Leases Receivable [Abstract] | | | | |
At December 31, 2016 | | 0 | | 0 |
Construction and Land [Member] | Pass [Member] | | | | |
Loans and Leases Receivable [Abstract] | | | | |
At December 31, 2016 | | 10,122,000 | | 19,859,000 |
Commercial Real Estate [Member] | | | | |
Loans and Leases Receivable [Abstract] | | | | |
At December 31, 2016 | | 219,253,000 | | 195,842,000 |
Impaired Loans Receivable, Recorded Investment [Abstract] | | | | |
Total Impaired, with Reserve, Recorded Investment | | 25,000 | | 34,000 |
Total Impaired, with no Reserve, Recorded Investment | | 437,000 | | 506,000 |
Total Impaired, Recorded Investment | | 462,000 | | 540,000 |
Impaired Loans Receivable, Unpaid Principal Balance [Abstract] | | | | |
Total Impaired with Reserve, Unpaid Principal Balance | | 434,000 | | 443,000 |
Total Impaired with no Reserve, Unpaid Principal Balance | | 437,000 | | 506,000 |
Total Impaired, Unpaid Principal Balance, Total | | 871,000 | | 949,000 |
Impaired Loans Receivable, Reserve [Abstract] | | | | |
Total Impaired with Reserve, Reserve | | 26,000 | | 34,000 |
Total Impaired with no Reserve, Reserve | | 0 | | 0 |
Impaired Loans, Reserve | | 26,000 | | 34,000 |
Impaired Loans Receivable, Cumulative Charge Offs [Abstract] | | | | |
Total Impaired with Reserve, Cumulative Charge-offs | | 409,000 | | 409,000 |
Total Impaired with no Reserve, Cumulative Charge-offs | | 0 | | 0 |
Total Impaired, Cumulative Charge-offs | | 409,000 | | 409,000 |
Impaired Loans Receivable, Average Recorded Investment [Abstract] | | | | |
Total Impaired with Reserve, Average Recorded Investment | | 30,000 | 43,000 | |
Total Impaired with no Reserve, Average Recorded Investment | | 438,000 | 513,000 | |
Total Impaired, Average Recorded Investment, Total | | 468,000 | 556,000 | |
Impaired Loans Receivable, Interest Paid YTD [Abstract] | | | | |
Total Impaired with Reserve, Interest Paid YTD | | 0 | 0 | |
Total Impaired with no Reserve, Interest Paid YTD | | 7,000 | 19,000 | |
Total Impaired, Interest Paid YTD | | 7,000 | 19,000 | |
Total impaired loans including troubled debt restructurings | | 313,000 | | 324,000 |
Commercial Real Estate [Member] | Substandard [Member] | | | | |
Loans and Leases Receivable [Abstract] | | | | |
At December 31, 2016 | | 462,000 | | 1,124,000 |
Commercial Real Estate [Member] | Watch [Member] | | | | |
Loans and Leases Receivable [Abstract] | | | | |
At December 31, 2016 | | 392,000 | | 295,000 |
Commercial Real Estate [Member] | Pass [Member] | | | | |
Loans and Leases Receivable [Abstract] | | | | |
At December 31, 2016 | | 218,399,000 | | 194,423,000 |
Consumer [Member] | | | | |
Loans and Leases Receivable [Abstract] | | | | |
At December 31, 2016 | | 378,000 | | 255,000 |
Impaired Loans Receivable, Recorded Investment [Abstract] | | | | |
Total Impaired, with Reserve, Recorded Investment | | 0 | | 0 |
Total Impaired, with no Reserve, Recorded Investment | | 0 | | 0 |
Total Impaired, Recorded Investment | | 0 | | 0 |
Impaired Loans Receivable, Unpaid Principal Balance [Abstract] | | | | |
Total Impaired with Reserve, Unpaid Principal Balance | | 0 | | 0 |
Total Impaired with no Reserve, Unpaid Principal Balance | | 0 | | 0 |
Total Impaired, Unpaid Principal Balance, Total | | 0 | | 0 |
Impaired Loans Receivable, Reserve [Abstract] | | | | |
Total Impaired with Reserve, Reserve | | 0 | | 0 |
Total Impaired with no Reserve, Reserve | | 0 | | 0 |
Impaired Loans, Reserve | | 0 | | 0 |
Impaired Loans Receivable, Cumulative Charge Offs [Abstract] | | | | |
Total Impaired with Reserve, Cumulative Charge-offs | | 0 | | 0 |
Total Impaired with no Reserve, Cumulative Charge-offs | | 0 | | 0 |
Total Impaired, Cumulative Charge-offs | | 0 | | 0 |
Impaired Loans Receivable, Average Recorded Investment [Abstract] | | | | |
Total Impaired with Reserve, Average Recorded Investment | | 0 | 0 | |
Total Impaired with no Reserve, Average Recorded Investment | | 0 | 0 | |
Total Impaired, Average Recorded Investment, Total | | 0 | 0 | |
Impaired Loans Receivable, Interest Paid YTD [Abstract] | | | | |
Total Impaired with Reserve, Interest Paid YTD | | 0 | 0 | |
Total Impaired with no Reserve, Interest Paid YTD | | 0 | 0 | |
Total Impaired, Interest Paid YTD | | 0 | 0 | |
Consumer [Member] | Substandard [Member] | | | | |
Loans and Leases Receivable [Abstract] | | | | |
At December 31, 2016 | | 0 | | 0 |
Consumer [Member] | Watch [Member] | | | | |
Loans and Leases Receivable [Abstract] | | | | |
At December 31, 2016 | | 0 | | 0 |
Consumer [Member] | Pass [Member] | | | | |
Loans and Leases Receivable [Abstract] | | | | |
At December 31, 2016 | | 378,000 | | 255,000 |
Commercial Loans [Member] | | | | |
Loans and Leases Receivable [Abstract] | | | | |
At December 31, 2016 | | 31,145,000 | | 36,697,000 |
Impaired Loans Receivable, Recorded Investment [Abstract] | | | | |
Total Impaired, with Reserve, Recorded Investment | | 0 | | 0 |
Total Impaired, with no Reserve, Recorded Investment | | 26,000 | | 26,000 |
Total Impaired, Recorded Investment | | 26,000 | | 26,000 |
Impaired Loans Receivable, Unpaid Principal Balance [Abstract] | | | | |
Total Impaired with Reserve, Unpaid Principal Balance | | 0 | | 0 |
Total Impaired with no Reserve, Unpaid Principal Balance | | 26,000 | | 26,000 |
Total Impaired, Unpaid Principal Balance, Total | | 26,000 | | 26,000 |
Impaired Loans Receivable, Reserve [Abstract] | | | | |
Total Impaired with Reserve, Reserve | | 0 | | 0 |
Total Impaired with no Reserve, Reserve | | 0 | | 0 |
Impaired Loans, Reserve | | 0 | | 0 |
Impaired Loans Receivable, Cumulative Charge Offs [Abstract] | | | | |
Total Impaired with Reserve, Cumulative Charge-offs | | 0 | | 0 |
Total Impaired with no Reserve, Cumulative Charge-offs | | 0 | | 0 |
Total Impaired, Cumulative Charge-offs | | 0 | | 0 |
Impaired Loans Receivable, Average Recorded Investment [Abstract] | | | | |
Total Impaired with Reserve, Average Recorded Investment | | 0 | 0 | |
Total Impaired with no Reserve, Average Recorded Investment | | 26,000 | 26,000 | |
Total Impaired, Average Recorded Investment, Total | | 26,000 | 26,000 | |
Impaired Loans Receivable, Interest Paid YTD [Abstract] | | | | |
Total Impaired with Reserve, Interest Paid YTD | | 0 | 0 | |
Total Impaired with no Reserve, Interest Paid YTD | | 0 | 0 | |
Total Impaired, Interest Paid YTD | | 0 | $ 0 | |
Commercial Loans [Member] | Substandard [Member] | | | | |
Loans and Leases Receivable [Abstract] | | | | |
At December 31, 2016 | | 863,000 | | 1,585,000 |
Commercial Loans [Member] | Watch [Member] | | | | |
Loans and Leases Receivable [Abstract] | | | | |
At December 31, 2016 | | 1,015,000 | | 741,000 |
Commercial Loans [Member] | Pass [Member] | | | | |
Loans and Leases Receivable [Abstract] | | | | |
At December 31, 2016 | | $ 29,267,000 | | $ 34,371,000 |