Fair Value Measurements (Fair Value Hierarchy Table) (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Oil and natural gas derivative contracts - current	$ 199,431	$ 440,359
Oil and natural gas derivative contracts - long-term	0	66,187
Total Assets	199,431	506,546
Quoted Prices in Active Markets (Level 1)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Oil and natural gas derivative contracts - current	0	0
Oil and natural gas derivative contracts - long-term		0
Total Assets	0	0
Significant Other Observable Inputs (Level 2)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Oil and natural gas derivative contracts - current	116,557	283,238
Oil and natural gas derivative contracts - long-term		34,862
Total Assets	116,557	318,100
Significant Unobservable Inputs (Level 3)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Oil and natural gas derivative contracts - current	82,874	157,121
Oil and natural gas derivative contracts - long-term		31,325
Total Assets	$ 82,874	$ 188,446