Acquisitions and Divestitures (Details) (USD $)	3 Months Ended	9 Months Ended
	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Assets:
Current		$ (665,000)
Noncurrent		54,000
Oil and natural gas properties		854,835,000
Total assets acquired		854,224,000
Liabilities:
Current liabilities		(3,718,000)
Asset retirement obligations		6,966,000
Total liabilities assumed		3,248,000
Net assets acquired		850,976,000
Business Acquisition Pro Forma Results of Operations
Total revenue and other	249,211,000	1,542,797,000	823,718,000
Total operating expenses	168,114,000	594,942,000	460,805,000
Net income	10,803,000	639,871,000	144,367,000
Basic	$ 0.07	$ 3.65	$ 0.95
Diluted	$ 0.07	$ 3.64	$ 0.94
Acquisition of interests in Henry Savings LP and Henry Savings Management LLC
Business Acquisition [Line Items]
Acquisition closing date			May 27, 2010
Amount of cash paid to acquire the entity	330,000,000		330,000,000
Amount of receivable recorded	7,000,000		7,000,000
Total consideration transferred	323,000,000		323,000,000
Acquisition of interests in two wholly owned subsidiaries of HighMount Exploration and Production LLC
Business Acquisition [Line Items]
Acquisition closing date			April 30, 2010
Total consideration transferred	327,000,000		327,000,000
Acquisition of cetain oil and natural gas properties from certain affiliates of Merit Energy Company
Business Acquisition [Line Items]
Acquisition closing date			January 29, 2010
Amount of cash paid to acquire the entity	152,000,000		152,000,000
Amount of receivable recorded	1,000,000		1,000,000
Total consideration transferred	151,000,000		151,000,000
Business Acquisition Panther [Member]
Business Acquisition [Line Items]
Acquisition closing date		June 1, 2011
Total consideration transferred		222,000,000
Business Acquisition Williston Basin [Member]
Business Acquisition [Line Items]
Acquisition closing date		May 2, 2011, and May 11, 2011
Amount of cash paid to acquire the entity		154,000,000
Amount of payable recorded		2,000,000
Total consideration transferred		156,000,000
Business Acquisition Concho [Member]
Business Acquisition [Line Items]
Acquisition closing date		March 31, 2011
Amount of cash paid to acquire the entity		196,000,000
Amount of receivable recorded		2,000,000
Total consideration transferred		194,000,000
Business Acquisition Various [Member]
Business Acquisition [Line Items]
Total consideration transferred		39,000,000
Business Acquisition Terminated [Member]
Business Acquisition [Line Items]
Execution date of acquisition purchase and sale agreement			July 2010
Amount of deposit included in cash paid	9,000,000		9,000,000
Acquisition contract price			95,000,000
Business Acquisition Crownquest/Element [Member]
Business Acquisition [Line Items]
Acquisition closing date			August 16, 2010
Total consideration transferred	95,000,000		95,000,000
Business Acquisition SandRidge [Member]
Business Acquisition [Line Items]
Acquisition closing date		April 1, 2011, and April 5, 2011
Amount of cash paid to acquire the entity		239,000,000
Amount of payable recorded		1,000,000
Total consideration transferred		240,000,000
Business Acquisition Permian Basin - Pending [Member]
Business Acquisition [Line Items]
Execution date of acquisition purchase and sale agreement		September 20, 2011
Acquisition contract price		17,000,000
Business Acquisition Permian Basin - Pending II [Member]
Business Acquisition [Line Items]
Execution date of acquisition purchase and sale agreement		September 8, 2011
Acquisition contract price		$ 88,000,000