Securities - Securities Available for Sale in Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Fair value, Less than 12 months	$ 71,324	$ 128,603
Unrealized loss, Less than 12 months	(2,178)	(888)
Fair value, 12 months or more	169,748	92,819
Unrealized loss, 12 months or more	(8,104)	(1,272)
Total Fair value	241,072	221,422
Total, unrealized loss	(10,282)	(2,160)
U.S. Treasury and Government Sponsored Entities' Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, Less than 12 months	34,659	99,766
Unrealized loss, Less than 12 months	(1,351)	(734)
Fair value, 12 months or more	84,629	21,222
Unrealized loss, 12 months or more	(3,828)	(450)
Total Fair value	119,288	120,988
Total, unrealized loss	(5,179)	(1,184)
States of the U.S. and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, Less than 12 months	33,187
Unrealized loss, Less than 12 months	(694)
Fair value, 12 months or more	13,473	14,009
Unrealized loss, 12 months or more	(577)	(138)
Total Fair value	46,660	14,009
Total, unrealized loss	(1,271)	(138)
Mortgage-Backed GSE Securities: Residential [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, Less than 12 months	3,478	28,837
Unrealized loss, Less than 12 months	(133)	(154)
Fair value, 12 months or more	71,646	57,588
Unrealized loss, 12 months or more	(3,699)	(684)
Total Fair value	75,124	86,425
Total, unrealized loss	$ (3,832)	$ (838)