Retirement Plans - Actuarial Assumptions Used for Pension Plans & Postretirement Medical Plan (Detail)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Plans
Benefit Obligations:
Discount rate	2.20%	2.90%	3.90%
Rate of compensation increase	3.80%	3.80%	3.80%
Net Periodic Benefit Cost:
Discount rate, Service cost	3.20%	3.90%	3.90%
Discount rate, Interest cost	2.60%	3.40%	3.30%
Expected return on plan assets	6.70%	7.10%	7.20%
Rate of compensation increase	3.80%	3.80%	4.50%
Postretirement Medical Plan
Benefit Obligations:
Discount rate	1.90%	2.80%	3.90%
Initial health care trend rate	6.00%	6.50%	6.90%
Ultimate trend rate	4.50%	4.50%	4.50%
Year in which ultimate trend rate is reached	2038	2038	2038