Postretirement Benefit Plans - The Weighted Average Assumptions Used to Determine Net Periodic Benefit Costs (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
United States [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Discount rate	4.17%	4.39%	4.09%
Expected long-term return on plan assets	7.00%	7.00%	7.50%
United Kingdom [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Discount rate	2.60%	4.00%	3.50%
Expected long-term return on plan assets	3.77%	4.59%	4.66%