U.K. Pension Plan Net Periodic Service Cost (Details) - Pension Plan [Member] $ in Thousands, £ in Millions	3 Months Ended
	Mar. 31, 2018 GBP (£)	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 GBP (£)	Dec. 31, 2018 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost		$ 1,523	$ 2,047
Defined Benefit Plan, Contribution by Employer	£ 1.5	$ 2,100
Scenario, Forecast [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Expected Future Benefit Payment, Remainder of Fiscal Year				£ 6.6	$ 9,200
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%
Labor and Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		$ 3,872	3,688
Other loss, net [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost		2,556	2,441
Expected return on plan assets		(4,905)	(4,082)
Total included in other loss, net		$ (2,349)	$ (1,641)