Fair Value Measurements (Fair Value Hierarchy Table) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Oil derivative contracts - current asset	$ 1,859	$ 0
Total Assets	1,859
Oil derivative contracts - current liability	(114,512)	(99,061)
Oil derivative contracts - long-term liabilities	(1,876)	0
Total Liabilities	(116,388)	(99,061)
Quoted Prices in Active Markets (Level 1)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Oil derivative contracts - current asset	0
Total Assets	0
Oil derivative contracts - current liability	0	0
Oil derivative contracts - long-term liabilities	0
Total Liabilities	0	0
Significant Other Observable Inputs (Level 2)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Oil derivative contracts - current asset	1,859
Total Assets	1,859
Oil derivative contracts - current liability	(114,512)	(99,061)
Oil derivative contracts - long-term liabilities	(1,876)
Total Liabilities	(116,388)	(99,061)
Significant Unobservable Inputs (Level 3)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Oil derivative contracts - current asset	0
Total Assets	0
Oil derivative contracts - current liability	0	0
Oil derivative contracts - long-term liabilities	0
Total Liabilities	$ 0	$ 0