Derivative Instruments and Hedging Activities (Schedule of Derivative Instruments in Statement of Financial Position, Fair Value) (Details) - Not Designated as Hedging Instrument - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Offsetting Assets and Liabilities [Line Items]
Derivative Assets	$ 406	$ 103
Derivative Liabilities	(29,028)	(18,296)
Current Assets
Offsetting Assets and Liabilities [Line Items]
Derivative asset, fair value, gross asset	406	1,836
Derivative asset, fair value, gross liability	0	(1,733)
Derivative Assets	406	103
Current Liabilities
Offsetting Assets and Liabilities [Line Items]
Derivative liability, fair value, gross liability	(27,744)	(20,001)
Derivative liability, fair value, gross asset	0	1,733
Derivative Liabilities	(27,744)	(18,268)
Non-current Liabilities
Offsetting Assets and Liabilities [Line Items]
Derivative liability, fair value, gross liability	(1,284)	(28)
Derivative liability, fair value, gross asset	0	0
Derivative Liabilities	$ (1,284)	$ (28)