Pension Plans and Other Postretirement Benefits - Assumed Weighted-Average Rates Used to Determine Benefit Obligations (Detail)	Dec. 31, 2018	Dec. 31, 2017
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Discount rate	4.25%	3.50%
Pension Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Discount rate	4.25%	3.50%
Rate of increase in future compensation levels	4.31%	4.33%
Other Postretirement Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Discount rate	4.25%	3.50%