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Securities - Gross Unrealized Losses, Aggregated by Investment Category and Length of Time (Detail) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]    
Gross Unrealized Losses, Less Than Twelve Months $ (1,139) $ (625)
Fair Value, Less Than Twelve Months 57,554 66,400
Gross Unrealized Losses, Twelve Months and Over (4,777) (2,643)
Fair Value, Twelve Months and Over 115,550 106,840
U.S. Treasury [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Gross Unrealized Losses, Less Than Twelve Months (104) (36)
Fair Value, Less Than Twelve Months 6,864 6,924
Gross Unrealized Losses, Twelve Months and Over (273) (205)
Fair Value, Twelve Months and Over 13,927 14,054
U.S. Government Agencies [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Gross Unrealized Losses, Less Than Twelve Months (469) (314)
Fair Value, Less Than Twelve Months 14,664 27,328
Gross Unrealized Losses, Twelve Months and Over (2,390) (1,418)
Fair Value, Twelve Months and Over 61,646 53,139
Mortgage-backed Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Gross Unrealized Losses, Less Than Twelve Months (94) (70)
Fair Value, Less Than Twelve Months 5,640 7,149
Gross Unrealized Losses, Twelve Months and Over (1,507) (720)
Fair Value, Twelve Months and Over 27,975 28,080
State and Local Governments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Gross Unrealized Losses, Less Than Twelve Months (472) (205)
Fair Value, Less Than Twelve Months 30,386 24,999
Gross Unrealized Losses, Twelve Months and Over (607) (300)
Fair Value, Twelve Months and Over $ 12,002 $ 11,567