Securities - Available for Sale Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 224,382	$ 192,820
Gross Unrealized Gains	5,705	4,292
Gross Unrealized Losses	(668)	(1,248)
Total securities available for sale, Fair Value	229,419	195,864
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	30,972	37,406
Gross Unrealized Gains	178	117
Gross Unrealized Losses	(87)	(77)
Total securities available for sale, Fair Value	31,063	37,446
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	109,443	92,177
Gross Unrealized Gains	4,397	3,395
Gross Unrealized Losses	(225)	(574)
Total securities available for sale, Fair Value	113,615	94,998
Mortgage-backed Securities in Government Sponsored Entities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	83,486	62,756
Gross Unrealized Gains	793	483
Gross Unrealized Losses	(356)	(597)
Total securities available for sale, Fair Value	83,923	62,642
Total Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	223,901	192,339
Gross Unrealized Gains	5,368	3,995
Gross Unrealized Losses	(668)	(1,248)
Total securities available for sale, Fair Value	228,601	195,086
Equity Securities in Financial Institutions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	481	481
Gross Unrealized Gains	337	297
Total securities available for sale, Fair Value	$ 818	$ 778