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
Sep. 30, 2018
Dec. 31, 2017
Offsetting Assets and Liabilities [Line Items]    
Derivative asset $ 4,289 $ 406
Derivative liability (62,607) (29,028)
Current assets: Fair value of derivatives    
Offsetting Assets and Liabilities [Line Items]    
Derivative asset, fair value, gross asset 16,157 406
Derivative asset, fair value, gross liability (11,868) 0
Derivative asset 4,289 406
Long-term assets: Fair value of derivatives    
Offsetting Assets and Liabilities [Line Items]    
Derivative asset, fair value, gross asset 2,786  
Derivative asset, fair value, gross liability (2,786)  
Derivative asset 0  
Current liabilities: Fair value of derivatives    
Offsetting Assets and Liabilities [Line Items]    
Derivative liability, fair value, gross liability (59,035) (27,744)
Derivative liability, fair value, gross asset 11,868 0
Derivative liability (47,167) (27,744)
Long-term liabilities: Fair value of derivatives    
Offsetting Assets and Liabilities [Line Items]    
Derivative liability, fair value, gross liability (18,226) (1,284)
Derivative liability, fair value, gross asset 2,786 0
Derivative liability $ (15,440) $ (1,284)