Employee Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Change in plan assets
Fair value of plan assets at beginning of year	$ 44,437
Fair value of plan assets at end of year	51,660	$ 44,437
Other changes in plan assets and benefit obligations recognized in other comprehensive income
Total recognized in other comprehensive income	22,139	16,350
Defined Benefit Pension Plan [Member]
Change projected in benefit obligation
Benefit obligation at beginning of year	40,509	47,065
Service cost	1,103	1,411
Interest cost	1,892	1,481
Actuarial (gain)/loss	7,099	(8,263)
Benefits paid	(1,169)	(1,185)
Projected benefit obligation at end of year	49,434	40,509
Change in plan assets
Fair value of plan assets at beginning of year	44,437	48,422
Actual return (loss) on plan assets	8,392	(2,800)
Employer contributions	0	0
Benefits paid	(1,169)	(1,185)
Fair value of plan assets at end of year	51,660	44,437
(Unfunded) Funded status	2,226	3,928
Accumulated benefit obligation	$ 49,434	$ 40,509
Weighted-average assumptions as of December 31
Discount rate - Liability	3.71%	4.76%
Discount rate - Expense	4.76%	3.49%
Expected return on plan assets	7.50%	8.00%
Rate of compensation increase	4.00%	4.00%
Components of net periodic benefit cost
Service cost	$ 1,103	$ 1,411
Interest cost	1,892	1,481
Expected return on plan assets	(3,275)	(3,813)
Recognized prior service cost		(100)
Recognized net losses	916	904
Net periodic benefit cost (credit)	636	(117)
Other changes in plan assets and benefit obligations recognized in other comprehensive income
Amortization of prior service cost		100
Net (gain) loss	1,066	(2,554)
Total recognized in other comprehensive income	1,066	(2,454)
Total recognized in net periodic benefit cost and other comprehensive income	1,702	(2,571)
Supplemental Insurance/ Retirement Plan [Member]
Change projected in benefit obligation
Benefit obligation at beginning of year	40,405	42,579
Service cost	1,024	1,107
Interest cost	1,926	1,386
Actuarial (gain)/loss	7,537	(3,591)
Benefits paid	(916)	(1,076)
Projected benefit obligation at end of year	49,976	40,405
Change in plan assets
Benefits paid	(916)	(1,076)
(Unfunded) Funded status	(49,976)	(40,405)
Accumulated benefit obligation	$ 45,238	$ 36,984
Weighted-average assumptions as of December 31
Discount rate - Liability	3.71%	4.79%
Discount rate - Expense	4.79%	3.42%
Rate of compensation increase	4.00%	4.00%
Components of net periodic benefit cost
Service cost	$ 1,024	$ 1,107
Interest cost	1,926	1,386
Recognized prior service cost	114	114
Recognized net losses	435	706
Net periodic benefit cost (credit)	3,499	3,313
Other changes in plan assets and benefit obligations recognized in other comprehensive income
Amortization of prior service cost	(114)	(114)
Net (gain) loss	7,101	(4,298)
Total recognized in other comprehensive income	6,987	(4,412)
Total recognized in net periodic benefit cost and other comprehensive income	$ 10,486	$ (1,099)