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Securities - Gross Unrealized Losses, Aggregated by Investment Category and Length of Time (Detail) - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Gross Unrealized Losses, Less Than Twelve Months $ (1,086) $ (3,803)
Fair Value, Less Than Twelve Months 109,706 167,230
Gross Unrealized Losses, Twelve Months and Over (885) (7)
Fair Value, Twelve Months and Over 31,643 526
U.S. Treasury [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Gross Unrealized Losses, Less Than Twelve Months (114) (146)
Fair Value, Less Than Twelve Months 15,676 15,745
U.S. Government Agencies [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Gross Unrealized Losses, Less Than Twelve Months (554) (1,795)
Fair Value, Less Than Twelve Months 62,910 77,471
Gross Unrealized Losses, Twelve Months and Over (510)  
Fair Value, Twelve Months and Over 15,251  
Mortgage-backed Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Gross Unrealized Losses, Less Than Twelve Months (288) (879)
Fair Value, Less Than Twelve Months 22,344 36,474
Gross Unrealized Losses, Twelve Months and Over (273)  
Fair Value, Twelve Months and Over 10,360  
State and Local Governments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Gross Unrealized Losses, Less Than Twelve Months (130) (983)
Fair Value, Less Than Twelve Months 8,776 37,540
Gross Unrealized Losses, Twelve Months and Over (102) (7)
Fair Value, Twelve Months and Over $ 6,032 $ 526