Postretirement Benefits - Weighted Average Pension Plan Assumptions and Weighted Average Assumptions for Post-Retirement Benefit Plans Other Than Pensions (Detail) - Pension Benefits	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount Rate	3.75%	4.27%
Rate of Compensation Increase	4.37%	4.31%
Discount Rate	4.27%	4.64%	4.23%
Expected Long-Term Returns	6.96%	7.19%	6.96%
Rate of Compensation Increase	4.31%	4.33%	4.35%