Securities - Available for Sale Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 233,575	$ 226,550
Gross Unrealized Gains	2,938	4,734
Gross Unrealized Losses	(2,471)	(1,054)
Fair Value	234,042	230,230
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	29,989	30,450
Gross Unrealized Gains	89	100
Gross Unrealized Losses	(281)	(192)
Fair Value	29,797	30,358
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	115,754	114,002
Gross Unrealized Gains	2,570	4,226
Gross Unrealized Losses	(661)	(172)
Fair Value	117,663	118,056
Mortgage-backed Securities in Government Sponsored Entities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	87,832	82,098
Gross Unrealized Gains	279	408
Gross Unrealized Losses	(1,529)	(690)
Fair Value	$ 86,582	$ 81,816