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