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Derivative Instruments and Hedging Activities (Schedule of Derivative Instruments in Statement of Financial Position, Fair Value) (Details) - Not Designated as Hedging Instrument [Member] - USD ($)
$ in Thousands
Mar. 31, 2017
Dec. 31, 2016
Offsetting Assets and Liabilities [Line Items]    
Derivative Assets $ 6,032 $ 103
Derivative Liabilities (6,430) (18,296)
Current assets - Fair value of derivatives [Member]    
Offsetting Assets and Liabilities [Line Items]    
Derivative asset, fair value, gross asset 5,055 1,836
Derivative asset, fair value, gross liability (1,962) (1,733)
Derivative Assets 3,093 103
Long-term assets: Fair value of derivatives [Member]    
Offsetting Assets and Liabilities [Line Items]    
Derivative asset, fair value, gross asset 2,939  
Derivative Assets 2,939  
Current liabilities - Fair value of derivatives [Member]    
Offsetting Assets and Liabilities [Line Items]    
Derivative liability, fair value, gross liability (8,392) (20,001)
Derivative liability, fair value, gross asset 1,962 1,733
Derivative Liabilities $ (6,430) (18,268)
Non-current liabilities - Fair value of derivatives [Member]    
Offsetting Assets and Liabilities [Line Items]    
Derivative liability, fair value, gross liability   (28)
Derivative Liabilities   $ (28)