Pension Plans and Other Postretirement Benefits - Net Periodic Defined Benefit Cost for Defined Benefit Plans (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 5,128	$ 4,986	$ 15,385	$ 14,958
Interest cost on projected benefit obligation	15,468	17,855	46,404	53,565
Expected return on plan assets	(35,862)	(31,378)	(107,586)	(94,134)
Amortization of prior service cost (credit)	139	125	415	404
Amortization of net actuarial loss (gain)	22,254	13,950	66,763	42,998
Net periodic cost (benefit)	7,127	5,538	21,381	17,791
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	254	242	760	727
Interest cost on projected benefit obligation	328	436	984	1,306
Amortization of prior service cost (credit)	(1,185)	(1,175)	(3,554)	(3,544)
Amortization of net actuarial loss (gain)	(324)	(300)	(971)	(918)
Net periodic cost (benefit)	$ (927)	$ (797)	$ (2,781)	$ (2,429)