Comprehensive Income - Components of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss	$ (17,205)	$ (5,228)	$ (7,402)
Net Unrealized Loss on Available For Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss, before tax	(18,975)		(8,083)
Accumulated other comprehensive loss, tax effect	5,610		3,399
Accumulated other comprehensive loss	(13,365)		(4,684)
Unfunded Status of Supplemental Retirement Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss, before tax	(5,238)		(4,621)
Accumulated other comprehensive loss, tax effect	1,549		1,943
Accumulated other comprehensive loss	(3,689)		(2,678)
Joint Beneficiary Agreement Liability [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss, before tax	(151)		(40)
Accumulated other comprehensive loss	$ (151)		$ (40)