Fair Value Measurements (Fair Value Hierarchy Table) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Oil derivative contracts - current liability	$ (125,354)	$ (99,061)
Oil derivative contracts - long-term liabilities	(13,570)	0
Total Liabilities	(138,924)	(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	(120,298)	(99,061)
Oil derivative contracts - long-term liabilities	(12,214)
Total Liabilities	(132,512)	(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	(5,056)	0
Oil derivative contracts - long-term liabilities	(1,356)
Total Liabilities	$ (6,412)	$ 0