Securities - Gross Unrealized Losses and Fair Value of Company's Investments (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Fair Value less than 12 months	$ 65,696	$ 92,615
Fair value more than 12 months	308,074	398,164
Total fair value	373,770	490,779
Unrealized losses less than 12 months	(698)	(1,767)
Unrealized losses more than 12 months	(6,216)	(12,794)
Total unrealized losses	(6,914)	(14,561)
U.S. Treasury and Federal Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value more than 12 months	9,823	9,707
Total fair value	9,823	9,707
Unrealized losses more than 12 months	(92)	(208)
Total unrealized losses	(92)	(208)
State and Municipal [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value less than 12 months	65,265	75,163
Fair value more than 12 months	44,783	106,335
Total fair value	110,048	181,498
Unrealized losses less than 12 months	(697)	(1,628)
Unrealized losses more than 12 months	(2,500)	(4,316)
Total unrealized losses	(3,197)	(5,944)
Federal Agency Collateralized Mortgage Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value less than 12 months	431	6,450
Fair value more than 12 months	100,886	106,257
Total fair value	101,317	112,707
Unrealized losses less than 12 months	(1)	(25)
Unrealized losses more than 12 months	(1,607)	(3,280)
Total unrealized losses	(1,608)	(3,305)
Federal Agency Mortgage-backed Pools [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value less than 12 months		5,739
Fair value more than 12 months	152,582	175,865
Total fair value	152,582	181,604
Unrealized losses less than 12 months		(39)
Unrealized losses more than 12 months	(2,017)	(4,990)
Total unrealized losses	$ (2,017)	(5,029)
Corporate Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value less than 12 months		5,263
Total fair value		5,263
Unrealized losses less than 12 months		(75)
Total unrealized losses		$ (75)