U.K. Pension Plan Net Periodic Service Cost (Details) - Pension Plan [Member] £ in Thousands, $ in Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2017 GBP (£)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 GBP (£)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 GBP (£)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 GBP (£)	Sep. 30, 2016 USD ($)	Dec. 31, 2017 GBP (£)	Dec. 31, 2017 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Service cost	£ 2,988	$ 3,909	£ 2,394	$ 3,143	£ 8,964	$ 11,434	£ 7,182	$ 10,006
Interest cost	1,978	2,588	2,227	2,923	5,934	7,569	6,681	9,308
Expected return on plan assets	(3,307)	(4,327)	(2,846)	(3,737)	(9,921)	(12,655)	(8,538)	(11,896)
Net periodic benefit cost	£ 1,659	$ 2,170	£ 1,775	$ 2,329	4,977	6,348	£ 5,325	$ 7,418
Defined Benefit Plan, Contribution by Employer					£ 4,500	$ 6,000
Scenario, Forecast [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Expected Future Benefit Payment, Remainder of Fiscal Year									£ 2,600	$ 3,500
U.K. Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
Company contribution % required in shared cost arrangement					60.00%	60.00%
Defined benefit plan, contribution percent by participant					40.00%	40.00%