Defined Benefit Plans - Schedule of Assumptions Used (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Weighted-Average Assumptions Used to Determine Net Cost for Years Ended December 31:
Expected long-term rate of return on plan assets	6.40%
Pension Benefits [Member]
Weighted-Average Assumptions Used to Determine Benefit Obligations at December 31:
Discount rate	3.80%	4.30%	4.60%
Rate of compensation increase		4.00%	4.00%
Weighted-Average Assumptions Used to Determine Net Cost for Years Ended December 31:
Discount rate	4.30%	4.60%	4.20%
Expected long-term rate of return on plan assets	6.40%	6.60%	6.80%
Rate of compensation increase		4.00%	4.00%
Postretirement Benefits [Member]
Weighted-Average Assumptions Used to Determine Benefit Obligations at December 31:
Discount rate	3.40%	3.40%	4.10%
Weighted-Average Assumptions Used to Determine Net Cost for Years Ended December 31:
Discount rate	3.40%	4.10%	3.50%