Postretirement Benefits - Components of Net Periodic Pension Costs and Post-Retirement Benefit Costs (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Retirement Benefits [Abstract]
Service cost	$ 6,116	$ 4,982
Interest cost	5,651	5,964
Expected return on assets	(7,390)	(6,966)
Amortization:
Prior service cost	158	158
Actuarial (gain) loss	3,924	1,894
Net periodic benefit cost	$ 8,459	$ 6,032