Retirement Plans - Information Pertaining to Activity in Supplemental Retirement Plans (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Change in benefit obligation:
Benefit obligation at beginning of year	$ (36,737)	$ (29,196)
Service cost	(2,225)	(879)	$ (973)
Interest cost	(1,014)	(1,131)	(949)
Actuarial (loss)/gain	640	(6,747)
Plan amendments	0	0
Benefits paid	1,336	1,216
Benefit obligation at end of year	(38,000)	(36,737)	$ (29,196)
Change in plan assets:
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Funded status	(38,000)	(36,737)
Unrecognized net obligation existing at January 1, 1986	0	1
Unrecognized net actuarial loss	1,380	11,333
Unrecognized prior service cost	(86)	(141)
Accumulated other comprehensive income	(1,294)	(11,193)
Accrued benefit cost	(38,000)	(36,737)
Accumulated benefit obligation	$ (36,298)	$ (35,981)