Securities - Components of Securities Available for Sale (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	$ 271,705	$ 348,099
Available-for-sale securities, Gross unrealized gains	1,016	225
Available-for-sale securities, Gross unrealized losses	(2,160)	(5,040)
Available-for-sale securities, Total fair value	270,561	343,284
U.S. Treasury and Government Sponsored Entities' Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	124,982	188,082
Available-for-sale securities, Gross unrealized gains	19	172
Available-for-sale securities, Gross unrealized losses	(1,184)	(2,221)
Available-for-sale securities, Total fair value	123,817	186,033
States of the U.S. and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	58,806	59,415
Available-for-sale securities, Gross unrealized gains	955	3
Available-for-sale securities, Gross unrealized losses	(138)	(1,661)
Available-for-sale securities, Total fair value	59,623	57,757
Mortgage-Backed GSE Securities: Residential [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	87,917	100,602
Available-for-sale securities, Gross unrealized gains	42	50
Available-for-sale securities, Gross unrealized losses	(838)	(1,158)
Available-for-sale securities, Total fair value	$ 87,121	$ 99,494