Derivative Financial Instruments Fair Value of Derivative Instrument (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
British pound forward contracts [Member] | Designated as Hedging Instrument [Member] | Other assets, net [Member]
Derivatives, Fair Value [Line Items]
Financial assets carried at fair value	$ 13,657	$ 26,359
British pound forward contracts [Member] | Designated as Hedging Instrument [Member] | Other long-term liabilities [Member]
Derivatives, Fair Value [Line Items]
Financial liabilities carried at fair value	829	17
Currency Swap [Member] | Not Designated as Hedging Instrument [Member] | Other assets, net [Member]
Derivatives, Fair Value [Line Items]
Financial assets carried at fair value	2,887	506
Currency Swap [Member] | Not Designated as Hedging Instrument [Member] | Prepaid expenses and other [Member]
Derivatives, Fair Value [Line Items]
Financial assets carried at fair value	5,775	174
Interest Rate Swap [Member] | Designated as Hedging Instrument [Member] | Accrued expenses [Member]
Derivatives, Fair Value [Line Items]
Financial liabilities carried at fair value	1,972	1,747
Interest Rate Swap [Member] | Designated as Hedging Instrument [Member] | Other long-term liabilities [Member]
Derivatives, Fair Value [Line Items]
Financial liabilities carried at fair value	12,410	13,411
Fair Value, Inputs, Level 2 [Member] | Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Financial assets carried at fair value	13,657	26,359
Financial liabilities carried at fair value	15,211	15,175
Fair Value, Inputs, Level 2 [Member] | Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Financial assets carried at fair value	$ 8,662	$ 680