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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
Mar. 31, 2019
Dec. 31, 2018
Offsetting Assets and Liabilities [Line Items]    
Derivative asset $ 11,757 $ 65,114
Derivative liability (31,533) (17,920)
Current assets: Fair value of derivatives    
Offsetting Assets and Liabilities [Line Items]    
Derivative asset, fair value, gross asset 20,540 78,091
Derivative asset, fair value, gross liability (9,168) (12,977)
Derivative asset 11,372 65,114
Long-term assets: Fair value of derivatives    
Offsetting Assets and Liabilities [Line Items]    
Derivative asset, fair value, gross asset 829  
Derivative asset, fair value, gross liability (444)  
Derivative asset 385  
Current liabilities: Fair value of derivatives    
Offsetting Assets and Liabilities [Line Items]    
Derivative liability, fair value, gross liability (33,718) (23,457)
Derivative liability, fair value, gross asset 9,168 12,977
Derivative liability (24,550) (10,480)
Long-term liabilities: Fair value of derivatives    
Offsetting Assets and Liabilities [Line Items]    
Derivative liability, fair value, gross liability (7,427) (7,440)
Derivative liability, fair value, gross asset 444 0
Derivative liability $ (6,983) $ (7,440)