Postretirement Benefits - Components of Net Periodic Pension Costs and Post-Retirement Benefit Costs (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Retirement Benefits [Abstract]
Service cost	$ 6,115	$ 4,983	$ 12,231	$ 9,965
Interest cost	5,647	5,964	11,298	11,928
Expected return on assets	(7,391)	(6,966)	(14,781)	(13,932)
Amortization:
Prior service cost	158	158	316	316
Actuarial (gain) loss	3,925	1,896	7,849	3,790
Net periodic benefit cost	$ 8,454	$ 6,035	$ 16,913	$ 12,067