Pension and Postretirement Benefit Plans - Weighted Average Assumptions Used to Determine Benefit Plans' Obligations (Detail)	Dec. 31, 2017	Dec. 31, 2016
Pension
Benefit obligations weighted-average assumptions:
Discount rate (percent)	3.50%	4.00%
Rates of increase in compensation levels (percent)	3.25%	3.50%
Postretirement
Benefit obligations weighted-average assumptions:
Discount rate (percent)	3.50%	4.00%
Rates of increase in compensation levels (percent)	0.00%	0.00%