Quarterly report pursuant to Section 13 or 15(d)

Derivative Instruments (Schedule of Derivative Instruments in Statement of Financial Position) (Details)

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Derivative Instruments (Schedule of Derivative Instruments in Statement of Financial Position) (Details) - USD ($)
$ in Thousands
Jun. 30, 2017
Dec. 31, 2016
General Discussion of Derivative Instruments and Hedging Activities [Abstract]    
Short-term derivative instruments - asset $ 46,416 $ 3,488
Long-term derivative instruments - asset 19,761 5,696
Short-term derivative instruments - liability 28,106 119,219
Long-term derivative instruments - liability $ 8,198 $ 26,759