Pension Plans and Other Postretirement Benefits - Net Periodic Defined Benefit Cost for Defined Benefit Plans (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 5,234	$ 5,258	$ 10,257	$ 9,972
Interest cost on projected benefit obligation	15,502	17,824	30,936	35,710
Expected return on plan assets	(35,774)	(31,281)	(71,724)	(62,756)
Amortization of prior service cost (credit)	151	154	276	279
Amortization of net actuarial loss (gain)	22,359	15,098	44,509	29,048
Net periodic cost (benefit)	7,472	7,053	14,254	12,253
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	258	262	506	485
Interest cost on projected benefit obligation	329	451	656	870
Amortization of prior service cost (credit)	(1,194)	(1,194)	(2,369)	(2,369)
Amortization of net actuarial loss (gain)	(347)	(318)	(647)	(618)
Net periodic cost (benefit)	$ (954)	$ (799)	$ (1,854)	$ (1,632)