Employee Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Change in plan assets
Fair value of plan assets at beginning of year	$ 51,660
Fair value of plan assets at end of year	56,433	$ 51,660
Other changes in plan assets and benefit obligations recognized in other comprehensive income
Total recognized in other comprehensive income	25,059	22,139
Defined Benefit Pension Plan [Member]
Change projected in benefit obligation
Benefit obligation at beginning of year	49,434	40,509
Service cost	1,377	1,103
Interest cost	1,801	1,892
Actuarial (gain)/loss	3,245	7,099
Benefits paid	(1,418)	(1,169)
Projected benefit obligation at end of year	54,439	49,434
Change in plan assets
Fair value of plan assets at beginning of year	51,660	44,437
Actual return on plan assets	6,191	8,392
Benefits paid	(1,418)	(1,169)
Fair value of plan assets at end of year	56,433	51,660
(Unfunded) Funded status	1,994	2,226
Accumulated benefit obligation	$ 54,439	$ 49,434
Weighted-average assumptions as of December 31
Discount rate — Liability	3.25%	3.71%
Discount rate — Expense	3.71%	4.76%
Expected return on plan assets	7.50%	7.50%
Rate of compensation increase	4.00%	4.00%
Components of net periodic benefit cost
Service cost	$ 1,377	$ 1,103
Interest cost	1,801	1,892
Expected return on plan assets	(3,808)	(3,275)
Recognized net losses	1,041	916
Net periodic benefit cost (credit)	411	636
Other changes in plan assets and benefit obligations recognized in other comprehensive income
Net (gain) loss	(179)	1,066
Total recognized in other comprehensive income	(179)	1,066
Total recognized in net periodic benefit cost and other comprehensive income	232	1,702
Supplemental Insurance/ Retirement Plan [Member]
Change projected in benefit obligation
Benefit obligation at beginning of year	49,976	40,405
Service cost	1,412	1,024
Interest cost	1,862	1,926
Prior sevice cost	501
Actuarial (gain)/loss	4,703	7,537
Benefits paid	(4,511)	(916)
Projected benefit obligation at end of year	53,943	49,976
Change in plan assets
Benefits paid	(4,511)	(916)
(Unfunded) Funded status	(53,943)	(49,976)
Accumulated benefit obligation	$ 48,767	$ 45,238
Weighted-average assumptions as of December 31
Discount rate — Liability	3.21%	3.71%
Discount rate — Expense	3.71%	4.79%
Rate of compensation increase	4.00%	4.00%
Components of net periodic benefit cost
Service cost	$ 1,412	$ 1,024
Interest cost	1,862	1,926
Recognized prior service cost	114	114
Recognized net losses	848	435
Net periodic benefit cost (credit)	4,236	3,499
Other changes in plan assets and benefit obligations recognized in other comprehensive income
Amortization of prior service cost	(114)	(114)
Net (gain) loss	3,809	7,101
Total recognized in other comprehensive income	3,695	6,987
Total recognized in net periodic benefit cost and other comprehensive income	$ 7,931	$ 10,486