Pension Plans and Other Postretirement Benefits - Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Net loss	$ 238,218	$ 126,618
Amortization of prior service (cost) credit	4,181	4,173	$ 4,172
Amortization of actuarial (loss) gain	(56,860)	(20,745)	$ (42,967)
Total recognized in other comprehensive income, pre-tax	185,539	110,046
Pension Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Net loss	235,847	127,305
Amortization of prior service (cost) credit	(557)	(557)
Amortization of actuarial (loss) gain	(58,096)	(21,992)
Total recognized in other comprehensive income, pre-tax	177,194	104,756
Other Postretirement Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Net loss	2,371	(687)
Amortization of prior service (cost) credit	4,738	4,730
Amortization of actuarial (loss) gain	1,236	1,247
Total recognized in other comprehensive income, pre-tax	$ 8,345	$ 5,290