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Loans - Schedule of Impaired Loans (Detail) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Mar. 31, 2017
Dec. 31, 2017
Receivables [Abstract]        
Impaired loans without a valuation allowance $ 999 $ 1,131 $ 1,099 $ 1,131
Impaired loans with a valuation allowance 607 614 694 614
Total impaired loans 1,606 1,745 1,793 1,745
Valuation allowance related to impaired loans 104 106 117 106
Total non-accrual loans 900 1,003 1,430 1,003
Total loans past-due ninety days or more and still accruing 0 0 0 0
Average investment in impaired loans $ 1,688 $ 2,160 $ 1,832 $ 1,885