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Securities - Gross Unrealized Losses, Aggregated by Investment Category and Length of Time (Detail) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]    
Gross Unrealized Losses, Less Than Twelve Months $ (1,404) $ (625)
Fair Value, Less Than Twelve Months 75,654 66,400
Gross Unrealized Losses, Twelve Months and Over (4,220) (2,643)
Fair Value, Twelve Months and Over 104,129 106,840
U.S. Treasury [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Gross Unrealized Losses, Less Than Twelve Months (87) (36)
Fair Value, Less Than Twelve Months 6,877 6,924
Gross Unrealized Losses, Twelve Months and Over (267) (205)
Fair Value, Twelve Months and Over 13,962 14,054
U.S. Government Agencies [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Gross Unrealized Losses, Less Than Twelve Months (619) (314)
Fair Value, Less Than Twelve Months 27,015 27,328
Gross Unrealized Losses, Twelve Months and Over (2,072) (1,418)
Fair Value, Twelve Months and Over 52,478 53,139
Mortgage-backed Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Gross Unrealized Losses, Less Than Twelve Months (173) (70)
Fair Value, Less Than Twelve Months 8,977 7,149
Gross Unrealized Losses, Twelve Months and Over (1,278) (720)
Fair Value, Twelve Months and Over 26,477 28,080
State and Local Governments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Gross Unrealized Losses, Less Than Twelve Months (525) (205)
Fair Value, Less Than Twelve Months 32,785 24,999
Gross Unrealized Losses, Twelve Months and Over (603) (300)
Fair Value, Twelve Months and Over $ 11,212 $ 11,567