Quarterly report pursuant to Section 13 or 15(d)

Derivative Instruments and Hedging Activities (Schedule of Derivative Instruments in Statement of Financial Position, Fair Value) (Details)

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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 - USD ($)
$ in Thousands
Jun. 30, 2017
Dec. 31, 2016
Current assets: Fair value of derivatives    
Offsetting Assets and Liabilities [Line Items]    
Derivative asset, fair value, gross asset $ 11,104 $ 1,836
Derivative asset, fair value, gross liability (1,863) (1,733)
Derivative asset 9,241 103
Long-term assets: Fair value of derivatives    
Offsetting Assets and Liabilities [Line Items]    
Derivative asset, fair value, gross asset 3,813  
Derivative asset, fair value, gross liability (9)  
Derivative asset 3,804  
Current liabilities: Fair value of derivatives    
Offsetting Assets and Liabilities [Line Items]    
Derivative liability, fair value, gross liability (4,106) (20,001)
Derivative liability, fair value, gross asset 1,863 1,733
Derivative liability 2,243 18,268
Long-term liabilities: Fair value of derivatives    
Offsetting Assets and Liabilities [Line Items]    
Derivative liability, fair value, gross liability (450) (28)
Derivative liability, fair value, gross asset 9 0
Derivative liability $ 441 $ 28