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, 2018
Dec. 31, 2017
General Discussion of Derivative Instruments and Hedging Activities [Abstract]    
Short-term derivative instruments - asset $ 20,745 $ 78,847
Long-term derivative instruments - asset 7,657 8,685
Short-term derivative instruments - liability 61,161 32,534
Long-term derivative instruments - liability $ 17,479 $ 2,989