Loans Receivable, Part IV (Details) - USD ($)
 3 Months Ended 12 Months Ended
Mar. 31, 2018
Dec. 31, 2015
Dec. 31, 2017
Loans Receivable, Recorded Investment [Line Items]      
At December 31, 2016 $ 1,314,672,000   $ 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 $ 863,000   1,016,000
Total Impaired, with no Reserve, Recorded Investment 5,726,000   8,160,000
Total Impaired, Recorded Investment 6,589,000   9,176,000
Impaired Loans Receivable, Unpaid Principal Balance [Abstract]      
Total Impaired with Reserve, Unpaid Principal Balance 1,272,000   1,425,000
Total Impaired with no Reserve, Unpaid Principal Balance 7,051,000   10,106,000
Total Impaired, Unpaid Principal Balance, Total 8,323,000   11,531,000
Impaired Loans Receivable, Reserve [Abstract]      
Total Impaired with Reserve, Reserve 169,000   155,000
Total Impaired with no Reserve, Reserve 0   0
Impaired Loans, Reserve 169,000   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 1,325,000   1,946,000
Total Impaired, Cumulative Charge-offs 1,734,000   2,355,000
Impaired Loans Receivable, Average Recorded Investment [Abstract]      
Total Impaired with Reserve, Average Recorded Investment 868,000 $ 1,039,000  
Total Impaired with no Reserve, Average Recorded Investment 5,787,000 8,451,000  
Total Impaired, Average Recorded Investment, Total 6,655,000 9,490,000  
Impaired Loans Receivable, Interest Paid YTD [Abstract]      
Total Impaired with Reserve, Interest Paid YTD 11,000 58,000  
Total Impaired with no Reserve, Interest Paid YTD 86,000 477,000  
Total Impaired, Interest Paid YTD 97,000 535,000  
Charge-offs recorded to reduce the unpaid principal balance 1,300,000    
Total impaired loans including troubled debt restructurings 4,962,000   5,082,000
Substandard [Member]      
Loans Receivable, Recorded Investment [Line Items]      
At December 31, 2016 11,905,000   11,563,000
Watch [Member]      
Loans Receivable, Recorded Investment [Line Items]      
At December 31, 2016 6,582,000   6,706,000
Pass [Member]      
Loans Receivable, Recorded Investment [Line Items]      
At December 31, 2016 1,296,185,000   1,273,545,000
Residential Real Estate [Member] | One-to-Four Family [Member]      
Loans Receivable, Recorded Investment [Line Items]      
At December 31, 2016 457,254,000   439,597,000
Impaired Loans Receivable, Recorded Investment [Abstract]      
Total Impaired, with Reserve, Recorded Investment 736,000   903,000
Total Impaired, with no Reserve, Recorded Investment 4,315,000   6,515,000
Total Impaired, Recorded Investment 5,051,000   7,418,000
Impaired Loans Receivable, Unpaid Principal Balance [Abstract]      
Total Impaired with Reserve, Unpaid Principal Balance 736,000   903,000
Total Impaired with no Reserve, Unpaid Principal Balance 4,780,000   7,604,000
Total Impaired, Unpaid Principal Balance, Total 5,516,000   8,507,000
Impaired Loans Receivable, Reserve [Abstract]      
Total Impaired with Reserve, Reserve 84,000   77,000
Total Impaired with no Reserve, Reserve 0   0
Impaired Loans, Reserve 84,000   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 465,000   1,089,000
Total Impaired, Cumulative Charge-offs 465,000   1,089,000
Impaired Loans Receivable, Average Recorded Investment [Abstract]      
Total Impaired with Reserve, Average Recorded Investment 738,000 913,000  
Total Impaired with no Reserve, Average Recorded Investment 4,364,000 6,796,000  
Total Impaired, Average Recorded Investment, Total 5,102,000 7,709,000  
Impaired Loans Receivable, Interest Paid YTD [Abstract]      
Total Impaired with Reserve, Interest Paid YTD 9,000 52,000  
Total Impaired with no Reserve, Interest Paid YTD 64,000 359,000  
Total Impaired, Interest Paid YTD 73,000 411,000  
Total impaired loans including troubled debt restructurings 3,851,000   3,896,000
Residential Real Estate [Member] | One-to-Four Family [Member] | Substandard [Member]      
Loans Receivable, Recorded Investment [Line Items]      
At December 31, 2016 7,953,000   7,581,000
Residential Real Estate [Member] | One-to-Four Family [Member] | Watch [Member]      
Loans Receivable, Recorded Investment [Line Items]      
At December 31, 2016 4,774,000   4,939,000
Residential Real Estate [Member] | One-to-Four Family [Member] | Pass [Member]      
Loans Receivable, Recorded Investment [Line Items]      
At December 31, 2016 444,527,000   427,077,000
Residential Real Estate [Member] | Multi Family [Member]      
Loans Receivable, Recorded Investment [Line Items]      
At December 31, 2016 571,690,000   578,440,000
Impaired Loans Receivable, Recorded Investment [Abstract]      
Total Impaired, with Reserve, Recorded Investment 0   0
Total Impaired, with no Reserve, Recorded Investment 1,179,000   1,007,000
Total Impaired, Recorded Investment 1,179,000   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 2,039,000   1,864,000
Total Impaired, Unpaid Principal Balance, Total 2,039,000   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 860,000   857,000
Total Impaired, Cumulative Charge-offs 860,000   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 1,190,000 1,005,000  
Total Impaired, Average Recorded Investment, Total 1,190,000 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 21,000 94,000  
Total Impaired, Interest Paid YTD 21,000 94,000  
Total impaired loans including troubled debt restructurings 793,000   815,000
Residential Real Estate [Member] | Multi Family [Member] | Substandard [Member]      
Loans Receivable, Recorded Investment [Line Items]      
At December 31, 2016 1,179,000   1,135,000
Residential Real Estate [Member] | Multi Family [Member] | Watch [Member]      
Loans Receivable, Recorded Investment [Line Items]      
At December 31, 2016 329,000   330,000
Residential Real Estate [Member] | Multi Family [Member] | Pass [Member]      
Loans Receivable, Recorded Investment [Line Items]      
At December 31, 2016 570,182,000   576,975,000
Residential Real Estate [Member] | Home Equity [Member]      
Loans Receivable, Recorded Investment [Line Items]      
At December 31, 2016 21,012,000   21,124,000
Impaired Loans Receivable, Recorded Investment [Abstract]      
Total Impaired, with Reserve, Recorded Investment 97,000   79,000
Total Impaired, with no Reserve, Recorded Investment 56,000   106,000
Total Impaired, Recorded Investment 153,000   185,000
Impaired Loans Receivable, Unpaid Principal Balance [Abstract]      
Total Impaired with Reserve, Unpaid Principal Balance 97,000   79,000
Total Impaired with no Reserve, Unpaid Principal Balance 56,000   106,000
Total Impaired, Unpaid Principal Balance, Total 153,000   185,000
Impaired Loans Receivable, Reserve [Abstract]      
Total Impaired with Reserve, Reserve 55,000   44,000
Total Impaired with no Reserve, Reserve 0   0
Impaired Loans, Reserve 55,000   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 98,000 83,000  
Total Impaired with no Reserve, Average Recorded Investment 57,000 111,000  
Total Impaired, Average Recorded Investment, Total 155,000 194,000  
Impaired Loans Receivable, Interest Paid YTD [Abstract]      
Total Impaired with Reserve, Interest Paid YTD 2,000 6,000  
Total Impaired with no Reserve, Interest Paid YTD 1,000 5,000  
Total Impaired, Interest Paid YTD 3,000 11,000  
Total impaired loans including troubled debt restructurings     47,000
Residential Real Estate [Member] | Home Equity [Member] | Substandard [Member]      
Loans Receivable, Recorded Investment [Line Items]      
At December 31, 2016 153,000   138,000
Residential Real Estate [Member] | Home Equity [Member] | Watch [Member]      
Loans Receivable, Recorded Investment [Line Items]      
At December 31, 2016 472,000   401,000
Residential Real Estate [Member] | Home Equity [Member] | Pass [Member]      
Loans Receivable, Recorded Investment [Line Items]      
At December 31, 2016 20,387,000   20,585,000
Construction and Land [Member]      
Loans Receivable, Recorded Investment [Line Items]      
At December 31, 2016 19,151,000   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 Receivable, Recorded Investment [Line Items]      
At December 31, 2016 0   0
Construction and Land [Member] | Watch [Member]      
Loans Receivable, Recorded Investment [Line Items]      
At December 31, 2016 0   0
Construction and Land [Member] | Pass [Member]      
Loans Receivable, Recorded Investment [Line Items]      
At December 31, 2016 19,151,000   19,859,000
Commercial Real Estate [Member]      
Loans Receivable, Recorded Investment [Line Items]      
At December 31, 2016 211,476,000   195,842,000
Impaired Loans Receivable, Recorded Investment [Abstract]      
Total Impaired, with Reserve, Recorded Investment 30,000   34,000
Total Impaired, with no Reserve, Recorded Investment 150,000   506,000
Total Impaired, Recorded Investment 180,000   540,000
Impaired Loans Receivable, Unpaid Principal Balance [Abstract]      
Total Impaired with Reserve, Unpaid Principal Balance 439,000   443,000
Total Impaired with no Reserve, Unpaid Principal Balance 150,000   506,000
Total Impaired, Unpaid Principal Balance, Total 589,000   949,000
Impaired Loans Receivable, Reserve [Abstract]      
Total Impaired with Reserve, Reserve 30,000   34,000
Total Impaired with no Reserve, Reserve 0   0
Impaired Loans, Reserve 30,000   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 32,000 43,000  
Total Impaired with no Reserve, Average Recorded Investment 150,000 513,000  
Total Impaired, Average Recorded Investment, Total 182,000 556,000  
Impaired Loans Receivable, Interest Paid YTD [Abstract]      
Total Impaired with Reserve, Interest Paid YTD 0 0  
Total Impaired with no Reserve, Interest Paid YTD 0 19,000  
Total Impaired, Interest Paid YTD 0 19,000  
Total impaired loans including troubled debt restructurings 318,000   324,000
Commercial Real Estate [Member] | Substandard [Member]      
Loans Receivable, Recorded Investment [Line Items]      
At December 31, 2016 1,047,000   1,124,000
Commercial Real Estate [Member] | Watch [Member]      
Loans Receivable, Recorded Investment [Line Items]      
At December 31, 2016 288,000   295,000
Commercial Real Estate [Member] | Pass [Member]      
Loans Receivable, Recorded Investment [Line Items]      
At December 31, 2016 210,141,000   194,423,000
Consumer [Member]      
Loans Receivable, Recorded Investment [Line Items]      
At December 31, 2016 240,000   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 Receivable, Recorded Investment [Line Items]      
At December 31, 2016 0   0
Consumer [Member] | Watch [Member]      
Loans Receivable, Recorded Investment [Line Items]      
At December 31, 2016 0   0
Consumer [Member] | Pass [Member]      
Loans Receivable, Recorded Investment [Line Items]      
At December 31, 2016 240,000   255,000
Commercial Loans [Member]      
Loans Receivable, Recorded Investment [Line Items]      
At December 31, 2016 33,849,000   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 Receivable, Recorded Investment [Line Items]      
At December 31, 2016 1,573,000   1,585,000
Commercial Loans [Member] | Watch [Member]      
Loans Receivable, Recorded Investment [Line Items]      
At December 31, 2016 719,000   741,000
Commercial Loans [Member] | Pass [Member]      
Loans Receivable, Recorded Investment [Line Items]      
At December 31, 2016 $ 31,557,000   $ 34,371,000