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Securities - Gross Unrealized Losses, Aggregated by Investment Category and Length of Time (Detail) - USD ($)
$ in Thousands
Dec. 31, 2016
Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]    
Gross Unrealized Losses, Less Than Twelve Months $ (3,803) $ (891)
Fair Value, Less Than Twelve Months 167,230 106,449
Gross Unrealized Losses, Twelve Months and Over (7) (451)
Fair Value, Twelve Months and Over 526 25,293
U.S. Treasury [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Gross Unrealized Losses, Less Than Twelve Months (146) (142)
Fair Value, Less Than Twelve Months 15,745 23,241
Gross Unrealized Losses, Twelve Months and Over   (167)
Fair Value, Twelve Months and Over   10,195
U.S. Government Agencies [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Gross Unrealized Losses, Less Than Twelve Months (1,795) (635)
Fair Value, Less Than Twelve Months 77,471 68,957
Gross Unrealized Losses, Twelve Months and Over   (200)
Fair Value, Twelve Months and Over   9,793
Mortgage-backed Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Gross Unrealized Losses, Less Than Twelve Months (879) (60)
Fair Value, Less Than Twelve Months 36,474 6,331
Gross Unrealized Losses, Twelve Months and Over   (56)
Fair Value, Twelve Months and Over   3,580
State and Local Governments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Gross Unrealized Losses, Less Than Twelve Months (983) (54)
Fair Value, Less Than Twelve Months 37,540 7,920
Gross Unrealized Losses, Twelve Months and Over (7) (28)
Fair Value, Twelve Months and Over $ 526 $ 1,725