Postretirement Benefit Plans - Components of Net Periodic Benefit Cost (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
United States [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Interest cost	$ 6,936	$ 6,424	$ 6,880
Expected return on plan assets	(9,523)	(8,828)	(8,423)
Amortization of net actuarial loss	2,687	5,109	3,573
Net periodic benefit cost	100	2,705	2,030
United Kingdom [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Interest cost	964	899	841
Expected return on plan assets	(1,303)	(941)	(888)
Amortization of net actuarial loss		288	49
Net periodic benefit cost	$ (339)	$ 246	$ 2