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