Financial Instruments (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Gain (Loss) on Foreign Currency Derivative Instruments Not Designated as Hedging Instruments	$ 5,700	$ 1,700
Not Designated as Hedging Instrument [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, Notional Amount	448,100		$ 394,800
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months	2,700
Cross Currency Swap [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Line Of Credit Facility Forecasted Borrowing Amount	$ 100,000
Derivative, Fixed Interest Rate	0.01%
Cross Currency Swap [Member] | Designated as Hedging Instrument [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Gain (Loss) on Interest Rate Cash Flow Hedge Ineffectiveness	$ 0	0
2.52% $50 Million Interest Rate Swap [Member] | Designated as Hedging Instrument [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Line Of Credit Facility Forecasted Borrowing Amount	$ 50,000
Derivative, Fixed Interest Rate	2.52%
Gain (Loss) on Interest Rate Cash Flow Hedge Ineffectiveness	$ 0	$ 0
2.25% $100 Million Interest Rate Swap [Member] | Designated as Hedging Instrument [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Line Of Credit Facility Forecasted Borrowing Amount	$ 100,000
Derivative, Fixed Interest Rate	2.25%