U.K. Pension Plan Net Periodic Service Cost (Details) - Pension Plan [Member] $ in Thousands, £ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 GBP (£)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2018 GBP (£)	Dec. 31, 2018 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 1,421	$ 2,170		$ 4,420	$ 6,348
Defined Benefit Plan, Contribution by Employer			£ 4.5	$ 6,000
Scenario, Forecast [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Expected Future Benefit Payment, Remainder of Fiscal Year						£ 2.9	$ 3,800
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	3,610	3,909		$ 11,235	11,434
Other income/(loss), net [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	2,383	2,588		7,416	7,569
Expected return on plan assets	(4,572)	(4,327)		(14,231)	(12,655)
Net Periodic Defined Benefits Expense (Reversal of Expense), Excluding Service Cost Component	$ (2,189)	$ (1,739)		$ (6,815)	$ (5,086)