Acquisitions and Divestitures (Details) (USD $)	3 Months Ended		6 Months Ended										12 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2010 Acquisition of interests in Henry Savings LP and Henry Savings Management LLC	Jun. 30, 2010 Acquisition of interests in two wholly owned subsidiaries of HighMount Exploration and Production LLC	Jun. 30, 2010 Acquisition of cetain oil and natural gas properties from certain affiliates of Merit Energy Company	Jun. 30, 2011 Business Acquisition Panther [Member]	Jun. 30, 2011 Business Acquisition Williston Basin [Member]	Jun. 30, 2011 Business Acquisition Permian Basin [Member]	Jun. 30, 2011 Business Acquisition Concho [Member]	Jun. 30, 2011 Business Acquisition Various [Member]	Dec. 31, 2010 Business Acquisition Terminated [Member]	Jul. 31, 2010 Business Acquisition Terminated [Member]
Business Acquisition [Line Items]
Effective date of acquisition													July 2010
Closing date of acquisition					May 27, 2010	April 30, 2010	January 29, 2010	June 1, 2011	May 2, 2011, and May 11, 2011	April 1, 2011, and April 5, 2011	March 31, 2011
Contract price of acquisition														$ 95,000,000
Amount of cash paid to acquire the entity									153,000,000	238,000,000	196,000,000
Amount of deposit included in cash paid					330,000,000	327,000,000	152,000,000							9,000,000
Amount of receivable recorded					7,000,000		1,000,000		3,000,000		2,000,000
Amount of payable recorded									1,000,000	1,000,000
Total consideration transferred					323,000,000		151,000,000	222,000,000	151,000,000	239,000,000	194,000,000	41,000,000
Assets:
Current	(1,481,000)		(1,481,000)
Noncurrent	54,000		54,000
Oil and natural gas properties	851,740,000		851,740,000
Total assets acquired	850,313,000		850,313,000
Liabilities:
Current liabilities	(3,819,000)		(3,819,000)
Asset retirement obligations	6,966,000		6,966,000
Total liabilities assumed	3,147,000		3,147,000
Net assets acquired	847,166,000		847,166,000
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Effective date of acquisition													July 2010
Total consideration transferred					323,000,000		151,000,000	222,000,000	151,000,000	239,000,000	194,000,000	41,000,000
Business Acquisition Pro Forma Results of Operations
Total revenues and other	521,386,000	305,436,000	423,314,000	574,507,000
Total operating expenses	201,393,000	152,638,000	383,409,000	292,691,000
Net income (loss)	$ 240,428,000	$ 65,980,000	$ (197,756,000)	$ 133,563,000
Basic	$ 1.36	$ 0.42	$ (1.16)	$ 0.89
Diluted	$ 1.35	$ 0.42	$ (1.16)	$ 0.89