Fair Value Measurements (Details) (USD $)	Jun. 30, 2011	Dec. 31, 2010
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments	$ 88,220,000	$ 93,020,000
Oil and natural gas derivative contracts	36,931,000	37,161,000
Oil and natural gas derivative contracts	(69,424,000)	(81,162,000)
Total	55,727,000	49,019,000
Quoted Prices in Active Markets (Level 1) [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments	88,220,000	93,020,000
Oil and natural gas derivative contracts	0	0
Oil and natural gas derivative contracts	0	0
Total	88,220,000	93,020,000
Significant Other Observable Inputs (Level 2) [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments	0	0
Oil and natural gas derivative contracts	30,293,000	20,683,000
Oil and natural gas derivative contracts	(69,424,000)	(81,162,000)
Total	(39,131,000)	(60,479,000)
Significant Unobservable Inputs (Level 3) [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments	0	0
Oil and natural gas derivative contracts	6,638,000	16,478,000
Oil and natural gas derivative contracts	0	0
Total	$ 6,638,000	$ 16,478,000