Postretirement Benefit Plans - Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income (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]
Net actuarial (gain) loss	$ 35,999	$ (18,539)	$ 4,622
Amortization of net actuarial loss	(2,687)	(5,109)	(3,573)
Total recognized in other comprehensive income	33,312	(23,648)	1,049
Total recognized in net periodic benefit cost and other comprehensive income	33,412	(20,943)	3,079
United Kingdom [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Net actuarial (gain) loss	2,577	(3,264)	3,111
Amortization of net actuarial loss		(288)	(49)
Total recognized in other comprehensive income	2,577	(3,552)	3,062
Total recognized in net periodic benefit cost and other comprehensive income	$ 2,238	$ (3,306)	$ 3,064