Employee Benefits - Summary of Defined Pension Plan and Supplemental Insurance Retirement Plan (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net Periodic Benefit Cost [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service cost	$ (739)	$ (307)
Net actuarial loss	(34,521)	(30,891)
Total	(35,260)	(31,198)
Defined Benefit Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	(3,245)	(7,099)
Total	(54,439)	(49,434)	$ (40,509)
Defined Benefit Pension Plan [Member] | Net Periodic Benefit Cost [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	(12,741)	(12,920)
Total	(12,741)	(12,920)
Supplemental Insurance/ Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service cost	114	114
Net actuarial loss	(4,703)	(7,537)
Total	(53,943)	(49,976)	$ (40,405)
Supplemental Insurance/ Retirement Plan [Member] | Net Periodic Benefit Cost [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service cost	(739)	(307)
Net actuarial loss	(21,780)	(17,971)
Total	$ (22,519)	$ (18,278)