Comprehensive Income - Components of Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), net of tax	$ 5,576	$ (5,222)
Net Unrealized Loss on Available for Sale Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss, before tax	13,423	3,387
Accumulated other comprehensive loss, tax effect	(3,968)	(1,001)
Accumulated other comprehensive income (loss), net of tax	9,455	2,386
Unfunded Status of Supplemental Retirement Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss, before tax	(7,193)	(11,193)
Accumulated other comprehensive loss, tax effect	2,126	3,309
Accumulated other comprehensive income (loss), net of tax	(5,067)	(7,884)
Joint Beneficiary Agreement Liability
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss, before tax	1,188	276
Accumulated other comprehensive loss, tax effect	0	0
Accumulated other comprehensive income (loss), net of tax	$ 1,188	$ 276