Comprehensive Income - Components of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss	$ (4,612)	$ (7,913)
Net Unrealized Loss on Available For Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss, before tax	(3,459)	(8,870)
Accumulated other comprehensive loss, tax effect	1,455	3,729
Accumulated other comprehensive loss	(2,004)	(5,141)
Unfunded Status of Supplemental Retirement Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss, before tax	(4,431)	(4,714)
Accumulated other comprehensive loss, tax effect	1,863	1,982
Accumulated other comprehensive loss	(2,568)	(2,732)
Joint Beneficiary Agreement Liability [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss, before tax	(40)	(40)
Accumulated other comprehensive loss	$ (40)	$ (40)