Financial Instruments Financial Instruments Textual (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Derivative [Line Items]
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months			$ 3,100
Designated as Hedging Instrument [Member] | Cross Currency Swap [Member]
Derivative [Line Items]
Line Of Credit Facility Forecasted Borrowing Amount	$ 100,000		$ 100,000
Derivative, Fixed Interest Rate	0.01%		0.01%
Gain (Loss) on Foreign Currency Cash Flow Hedge Ineffectiveness	$ 0		$ 0
Designated as Hedging Instrument [Member] | 2.52% $50 Million Interest Rate Swap [Member]
Derivative [Line Items]
Line Of Credit Facility Forecasted Borrowing Amount	$ 50,000		$ 50,000
Derivative, Fixed Interest Rate	2.52%		2.52%
Gain (Loss) on Interest Rate Cash Flow Hedge Ineffectiveness	$ 0		$ 0
Designated as Hedging Instrument [Member] | 2.25% $100 Million Interest Rate Swap [Member]
Derivative [Line Items]
Line Of Credit Facility Forecasted Borrowing Amount	$ 100,000		$ 100,000
Derivative, Fixed Interest Rate	2.25%		2.25%
Gain (Loss) on Interest Rate Cash Flow Hedge Ineffectiveness	$ 0		$ 0
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative, Notional Amount	407,100		407,100		$ 394,800
Gain (Loss) on Foreign Currency Derivative Instruments Not Designated as Hedging Instruments	$ 1,500	$ (100)	$ (4,200)	$ (1,900)