Derivatives (Fair Value of Derivative Instruments and Locations in Balance Sheets) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Current derivative asset	$ 127	$ 721
Non-current derivative asset	0	0
Total derivatives assets	127	721
Current derivative liability	(18,555)	(7,763)
Non-current derivative liability	(910)	0
Total derivative liabilities	$ (19,465)	$ (7,763)