Pension and Other Postretirement Benefits - Schedule of Weighted Average Assumptions Used to Determine Benefit Obligations (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Discount rate	3.75%	4.20%
Rate of compensation increase	3.50%	3.50%
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Discount rate to calculate service cost	4.20%	4.29%	4.00%
Discount rate to calculate interest cost	3.60%	3.15%	4.00%
Expected long-term rate of return on plan assets	7.75%	7.75%	7.75%
Rate of compensation increase	3.50%	3.50%	3.50%