Acquisitions (Details) (USD $)	3 Months Ended		6 Months Ended				3 Months Ended	12 Months Ended		1 Months Ended	3 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010	Jan. 31, 2010 Class A Permian Basin Acquisitions	Mar. 31, 2011 Permian Basin Acquisitions	Dec. 31, 2010 Permian Basin Acquisitions	Nov. 30, 2010 Permian Basin Acquisitions	May 31, 2011 Wolfberry Acquisition A	Jun. 30, 2011 Wolfberry Acquisition	Jun. 30, 2011 Class A	Dec. 31, 2010 Class A
Acquisitions
Interests acquired in producing properties (in net acres)										6,000
Purchase price paid, net of customary post-closing adjustments			$ 145,461,000	$ 150,674,000						$ (129,400,000)
Initial purchase price										128,500,000
Customary post-closing adjustments										900,000
Percentage of operation of acquired properties (as a percent)										97.00%
Percentage of average working interest (as a percent)										93.00%
Percentage Of net revenue interest (as a percent)										70.00%
Revenues generated by acquired properties	230,760,000	151,525,000	418,149,000	299,332,000							1,400,000
Operating costs - oil and gas production	58,780,000	46,452,000	115,863,000	93,488,000							300,000
Consideration paid to seller:
Cash consideration								327,032,000		129,376,000
Recognized amounts of identifiable assets acquired and liabilities assumed:
Proved developed and undeveloped properties								332,214,000		129,631,000
Other assets acquired								342,000
Asset retirement								(3,498,000)		(119,000)
Other liabilities assumed								(333,000)		(136,000)
Deferred income tax liability								(647,000)
Total identifiable net assets								328,078,000		129,376,000
Number of separate acquisitions								3
Aggregate purchase price related to acquisitions								327,000,000
Common stock shares issued for financing acquisitions (in shares)						8,000,000						51,604,476	51,426,232
Aggregate principal amount of senior notes due in november 2020 issued for financing acquisitions	300,000,000		300,000,000		300,000,000				300,000,000
Interest rate of senior notes due in november 2020 (as a percent)									6.75%
Gain on purchase of oil and natural gas properties							1,000,000
Deferred income taxes			$ 19,572,000	$ 61,142,000			$ 700,000