Pension Plans and Other Postretirement Benefits - Net Periodic Benefit Cost for Defined Benefit Plans (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 19,944	$ 17,294	$ 20,346
Interest cost on benefit obligation	71,421	81,579	74,704
Expected return on plan assets	(125,512)	(122,139)	(123,127)
Amortization of prior service cost	557	557	557
Recognized net actuarial (gain) loss	58,096	21,992	43,793
Net periodic pension cost (benefit)	24,506	(717)	16,273
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	970	859	938
Interest cost on benefit obligation	1,741	2,344	2,293
Amortization of prior service cost	(4,738)	(4,730)	(4,729)
Recognized net actuarial (gain) loss	(1,236)	(1,247)	(826)
Net periodic pension cost (benefit)	$ (3,263)	$ (2,774)	$ (2,324)