Securities - Components of Available for Sale Portfolio (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	$ 359,313	$ 361,503
Available-for-sale securities, Gross unrealized gains	6,073	406
Available-for-sale securities, Gross unrealized losses	(17)	(4,239)
Total fair value	365,369	357,670
U.S. Treasury and Government Sponsored Entities' Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	208,369	221,500
Available-for-sale securities, Gross unrealized gains	4,529	159
Available-for-sale securities, Gross unrealized losses		(3,009)
Total fair value	212,898	218,650
States of the U.S. and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	35,294	10,848
Available-for-sale securities, Gross unrealized gains	463	192
Available-for-sale securities, Gross unrealized losses	(17)
Total fair value	35,740	11,040
Mortgage-Backed GSE Securities: Residential [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	115,650	129,155
Available-for-sale securities, Gross unrealized gains	1,081	55
Available-for-sale securities, Gross unrealized losses		(1,230)
Total fair value	$ 116,731	$ 127,980