Comprehensive Income - Components of Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Tax effect		$ (3,309)
Accumulated other comprehensive income (loss), net of tax	$ 12,280	(5,222)
Net unrealized gain (loss) on available for sale securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOCI before tax, attributable to parent	19,183	3,387
Tax effect	(5,671)	(1,001)
Accumulated other comprehensive income (loss), net of tax	13,512	2,386
Unfunded status of the supplemental retirement plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOCI before tax, attributable to parent	(1,294)	(11,193)
Tax effect	382	3,309
Accumulated other comprehensive income (loss), net of tax	(912)	(7,884)
Joint beneficiary agreement liability, net of tax
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOCI before tax, attributable to parent	$ (320)	$ 276