U.K. Pension Plan Net Periodic Benefit Cost (Details) - Pension Plan [Member] - Foreign Plan [Member] - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 11,345	$ 15,401	$ 12,980
Interest cost	8,812	10,194	12,074
Expected return on plan assets	(12,509)	(17,046)	(15,434)
Net periodic benefit cost	$ 7,648	$ 8,549	$ 9,620