Pensions and Other Post-retirement Benefits - Schedule of Assumptions Used to Determine Benefit Obligations and Net Periodic Benefit Cost (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Assumptions used to determine benefit obligations, average discount rate	3.34%	3.67%
Assumptions used to determine benefit obligations, rate of compensation increase	3.00%	2.99%
Assumptions used to determine net periodic benefit cost, average discount rate	3.67%	3.92%
Assumptions used to determine net periodic benefit cost, expected return on plan assets	8.04%	8.18%
Assumptions used to determine net periodic benefit cost, rate of compensation increases	2.99%	3.06%
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Assumptions used to determine benefit obligations, average discount rate	3.57%	4.05%
Assumptions used to determine net periodic benefit cost, average discount rate	4.05%	4.20%