Pensions and Other Post-retirement Benefits - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Estimated contributions by employer to pension plans	$ 7,700
Percentage increase in health care benefit cost		6.50%
Percentage decrease in health care benefits for successive year	0.20%	0.50%
Estimated future percentage of health care benefits	4.50%	4.50%
Increase in costs of covered health care benefits	5.90%
Defined contribution pension plan expense	$ 10,600	$ 8,300	$ 9,000
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)	10,082	3,382	5,901
Service cost	12,094	10,342	11,125
Estimated benefit payments, year one	26,100
Estimated benefit payments, year two	27,000
Estimated benefit payments, year three	28,200
Estimated benefit payments, year four	28,800
Estimated benefit payments, year five	29,900
Estimated benefit payments, five years thereafter	155,900
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)	1,863	1,814	1,509
Service cost	396	$ 354	$ 369
Estimated benefit payments, year one	2,800
Estimated benefit payments, year two	2,700
Estimated benefit payments, year three	2,300
Estimated benefit payments, year four	2,400
Estimated benefit payments, year five	2,200
Estimated benefit payments, five years thereafter	$ 10,300