Fair Value Measurements (Fair Value Hierarchy) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Oil derivative contracts - current	$ (99,061)	$ (69,279)
Total Liabilities	(99,061)	(69,279)
Quoted Prices in Active Markets (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Oil derivative contracts - current	0	0
Total Liabilities	0	0
Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Oil derivative contracts - current	(99,061)	(68,753)
Total Liabilities	(99,061)	(68,753)
Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Oil derivative contracts - current	0	(526)
Total Liabilities	$ 0	$ (526)