Derivative Financial Instruments Effect of Cash Flow Hedges in OCI (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
British pound forward contracts [Member]
Derivatives [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net	$ (8,300)	$ 14,700
Other Comprehensive Income (Loss) [Member] | Cash Flow Hedging [Member]
Derivatives [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net	1,870	(5,666)	$ (3,837)
Other Comprehensive Income (Loss) [Member] | Cash Flow Hedging [Member] | Designated as Hedging Instrument [Member] | Interest rate swap agreement [Member]
Derivatives [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net	372	(1,676)	(4,749)
Other Comprehensive Income (Loss) [Member] | Cash Flow Hedging [Member] | Designated as Hedging Instrument [Member] | British pound forward contracts [Member]
Derivatives [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net	1,498	(3,990)	$ 912
Intercompany Loan [Member]
Derivatives [Line Items]
Foreign Currency Transaction Gain (Loss), Net of Tax	$ 9,800	$ (18,700)