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