Fair Value Measurements (Fair Value Hierarchy Table) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Oil derivative contracts - current liability	$ (145,254)	$ (99,061)
Oil derivative contracts - long-term liabilities	(10,704)	0
Total Liabilities	(155,958)	(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 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 liability	(144,554)	(99,061)
Oil derivative contracts - long-term liabilities	(10,236)
Total Liabilities	(154,790)	(99,061)
Significant Unobservable Inputs (Level 3)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Oil derivative contracts - current liability	(700)	0
Oil derivative contracts - long-term liabilities	(468)
Total Liabilities	$ (1,168)	$ 0