Assumptions Used to Determine Benefit Obligations (Detail)	Dec. 31, 2018	Dec. 31, 2017
Pension benefits
Defined Benefit Plan Disclosure [Line Items]
Discount rate	3.90%	3.40%
Long-term rate of compensation increase	4.50%	4.50%
Other postretirement benefits
Defined Benefit Plan Disclosure [Line Items]
Discount rate	3.90%	3.40%
Long-term rate of compensation increase	4.50%	4.50%