Securities - Securities Available for Sale in Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Fair value, Less than 12 months	$ 149,064	$ 128,603
Unrealized loss, Less than 12 months	(4,635)	(888)
Fair value, 12 months or more	86,188	92,819
Unrealized loss, 12 months or more	(3,511)	(1,272)
Total Fair value	235,252	221,422
Total, unrealized loss	(8,146)	(2,160)
States of the U.S. and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, Less than 12 months	22,769
Unrealized loss, Less than 12 months	(419)
Fair value, 12 months or more	13,669	14,009
Unrealized loss, 12 months or more	(415)	(138)
Total Fair value	36,438	14,009
Total, unrealized loss	(834)	(138)
U.S. Treasury and Government Sponsored Entities' Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, Less than 12 months	99,820	99,766
Unrealized loss, Less than 12 months	(3,238)	(734)
Fair value, 12 months or more	20,618	21,222
Unrealized loss, 12 months or more	(963)	(450)
Total Fair value	120,438	120,988
Total, unrealized loss	(4,201)	(1,184)
Mortgage-Backed GSE Securities: Residential [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, Less than 12 months	26,475	28,837
Unrealized loss, Less than 12 months	(978)	(154)
Fair value, 12 months or more	51,901	57,588
Unrealized loss, 12 months or more	(2,133)	(684)
Total Fair value	78,376	86,425
Total, unrealized loss	$ (3,111)	$ (838)