Pension Plans and Other Postretirement Benefits - Assumed Weighted-Average Rates Used to Determine Benefit Obligations (Detail)	Dec. 31, 2020	Dec. 31, 2019
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Discount rate	2.50%	3.25%
Pension Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Discount rate	2.50%	3.25%
Rate of increase in future compensation levels	3.37%	4.29%
Other Postretirement Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Discount rate	2.50%	3.25%