Pension Plans and Other Postretirement Benefits - Net Periodic Defined Benefit Cost for Defined Benefit Plans (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 5,023	$ 4,714
Interest cost on projected benefit obligation	15,434	17,886
Expected return on plan assets	(35,950)	(31,475)
Amortization of prior service cost (credit)	125	125
Amortization of net actuarial loss (gain)	22,150	13,950
Net periodic cost (benefit)	6,782	5,200
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	248	223
Interest cost on projected benefit obligation	327	419
Amortization of prior service cost (credit)	(1,175)	(1,175)
Amortization of net actuarial loss (gain)	(300)	(300)
Net periodic cost (benefit)	$ (900)	$ (833)