Pension Plans and Other Postretirement Benefits - Net Periodic Benefit Cost for Defined Benefit Plans (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 17,294	$ 20,346	$ 20,193
Interest cost on benefit obligation	81,579	74,704	79,270
Expected return on plan assets	(122,139)	(123,127)	(108,524)
Amortization of prior service cost	557	557	557
Recognized net actuarial (gain) loss	21,992	43,793	29,263
Net periodic pension cost (benefit)	(717)	16,273	20,759
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	859	938	1,172
Interest cost on benefit obligation	2,344	2,293	3,716
Amortization of prior service cost	(4,730)	(4,729)	(1,359)
Recognized net actuarial (gain) loss	(1,247)	(826)	(988)
Net periodic pension cost (benefit)	$ (2,774)	$ (2,324)	$ 2,541