Postretirement Benefits - Components of Net Periodic Pension Costs and Post-Retirement Benefit Costs (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Retirement Benefits [Abstract]
Service cost	$ 6,116	$ 4,982	$ 18,347	$ 14,947
Interest cost	5,653	5,964	16,951	17,892
Expected return on assets	(7,390)	(6,966)	(22,171)	(20,898)
Amortization:
Prior service cost	158	158	474	474
Actuarial (gain) loss	3,921	1,894	11,770	5,684
Net periodic benefit cost	$ 8,458	$ 6,032	$ 25,371	$ 18,099