Fair Value Measurements (Fair Value of Assets and Liabilities Measured on Recurring Basis) (Details) - Recurring Basis - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Assets
Derivative financial instruments	$ 11,757	$ 65,114
Liabilities
Derivative financial instruments	(31,533)	(17,920)
Net Derivative Fair Value	(19,776)	47,194
Level 1
Assets
Derivative financial instruments	0	0
Liabilities
Derivative financial instruments	0	0
Net Derivative Fair Value	0	0
Level 2
Assets
Derivative financial instruments	11,757	65,114
Liabilities
Derivative financial instruments	(31,533)	(17,920)
Net Derivative Fair Value	(19,776)	47,194
Level 3
Assets
Derivative financial instruments	0	0
Liabilities
Derivative financial instruments	0	0
Net Derivative Fair Value	$ 0	$ 0