Annual report pursuant to Section 13 and 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
Dec. 31, 2017
Dec. 31, 2016
General Discussion of Derivative Instruments and Hedging Activities [Abstract]    
Short-term derivative instruments - asset $ 78,847 $ 3,488
Long-term derivative instruments 8,685 5,696
Short-term derivative instruments - liability 32,534 119,219
Long-term derivative instruments - liability $ 2,989 $ 26,759