Comprehensive Income - Components of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss	$ (5,228)	$ (7,913)
Net Unrealized Loss on Available For Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss, before tax	(3,409)	(8,870)
Accumulated other comprehensive loss, tax effect	1,008	3,729
Accumulated other comprehensive loss	(2,401)	(5,141)
Unfunded Status of Supplemental Retirement Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss, before tax	(5,352)	(4,714)
Accumulated other comprehensive loss, tax effect	2,250	1,982
Accumulated other comprehensive loss	(3,102)	(2,732)
Joint Beneficiary Agreement Liability [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss, before tax	(150)	(40)
Accumulated other comprehensive loss	$ (150)	$ (40)