Acquisitions and Divestitures (Details) (USD $)	12 Months Ended						0 Months Ended		3 Months Ended	12 Months Ended				0 Months Ended		0 Months Ended		12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Common Class A	Dec. 31, 2011 Common Class A	Jan. 31, 2010 Common Class A	May 25, 2011 Wolfberry Acquisition	Sep. 12, 2012 Brundage Canyon Acquisition acre	Mar. 31, 2011 Permian Basin Acquisitions	Dec. 31, 2010 Permian Basin Acquisitions property	Dec. 31, 2012 Senior Notes 6.75 Percent Due 2020	Dec. 31, 2011 Senior Notes 6.75 Percent Due 2020	Nov. 01, 2010 Senior Notes 6.75 Percent Due 2020	Jan. 31, 2012 Nevada Assets	Dec. 31, 2011 Nevada Assets	Apr. 13, 2012 2012 Wolfberry Acquisition acre Wells	May 25, 2011 2011 Wolfberry Acquisition acre	Dec. 31, 2011 2011 Wolfberry Acquisition	May 25, 2011 Oil and Gas Properties Wolfberry Acquisition	Dec. 31, 2010 Oil and Gas Properties Permian Basin Acquisitions	May 25, 2011 Asset Retirement Obligation Costs Wolfberry Acquisition	Dec. 31, 2010 Asset Retirement Obligation Costs Permian Basin Acquisitions
Acquisitions
Payments for (Proceeds from) Deposits on Real Estate Acquisitions	$ 3,300,000	$ (3,300,000)	$ 0
Asset retirement obligations	82,316,000	59,256,000													700,000
Acquisition of Oil and Gas Properties Acres Acquired								14,000								2,000	6,000
Number of wells acquired in acquisition																1
Purchase price paid, net of customary post-closing adjustments	78,313,000	158,090,000	334,409,000				128,398,000	39,600,000		327,032,000						14,900,000	128,400,000
Common Stock, Shares, Issued				52,428,423	52,067,994	8,000,000
Revenues generated by acquired properties	937,261,000	870,773,000	619,608,000							28,700,000								7,800,000
Recognized amounts of identifiable assets acquired and liabilities assumed:
Proved developed and undeveloped properties																			128,697,000	332,214,000
Other assets acquired										342,000
Asset retirement obligation																					(119,000)	(3,498,000)
Deferred income tax liability										(647,000)
Other liabilities assumed							(180,000)			(333,000)
Total identifiable net assets							128,398,000			328,078,000
Number of separate acquisitions (in properties)										3
Senior Notes											300,000,000	300,000,000	300,000,000
Interest rate on debt instrument (as a percent)											6.75%
Gain on purchase of oil and natural gas properties	0	1,046,000	0						1,000,000
Deferred income taxes	82,881,000	(149,279,000)	51,735,000						700,000
Transaction Costs on Acquisitions	0	0	2,635,000							2,600,000
Proceeds from sale of assets	17,307,000	0	0											15,600,000
Gain (Loss) on Sale of Oil and Gas Property														1,600,000
Assets Held-for-sale, Current	$ 0	$ 14,622,000