Acquisitions and Divestitures - Narrative (Details) (USD $)	3 Months Ended								12 Months Ended					1 Months Ended		3 Months Ended				1 Months Ended		1 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Nov. 13, 2012	May 18, 2011	Sep. 30, 2012 Bakken [Member]	Sep. 30, 2012 Brazos and Madison Counties [Member]	Dec. 31, 2012 Noble Energy, Inc. [Member]	Jul. 24, 2012 Noble Energy, Inc. [Member]	Sep. 17, 2012 Noble Energy, Inc. [Member] Granite Wash [Member] acre locations	Sep. 17, 2012 Noble Energy, Inc. [Member] Other Plays [Member] acre	Jun. 30, 2010 Beaver County, Oklahoma [Member]	Jun. 02, 2010 Beaver County, Oklahoma [Member] Unit acre	Jul. 20, 2011 Oklahoma And Lipscomb [Member] acre Unit	Aug. 31, 2011 Oklahoma Arkoma Woodford And Hartshorne Coal [Member] locations acre	Sep. 17, 2012 Western Oklahoma and Texas Panhandle [Member] Noble Energy, Inc. [Member] boe acre systems
Business Combination, Description [Abstract]
Payments to acquire oil and gas property																				$ 73,700,000		$ 12,300,000	$ 30,500,000
Purchase price allocation for proved properties																				48,700,000		8,400,000
Undeveloped leasehold																				25,000,000		3,900,000
Gas and oil acreage, developed and undeveloped, net																					45,000
Operating number of wells																					10
Acquired operated wells																						30
Acquired non-operated wells																						59
Acres acquired held by production																						12,000
Operating and non-operating number of wells																							500
Oil And Gas Developed And Undeveloped Acreage Net																							55,000
Percentage of acre held for production																							96.00%
Land acquired (in acres)																		24,000	59,000					83,000
Total consideration given																								592,627,000
Proved developed reserves (BOE)																								44,000,000
Number of horizontal driling locations included in the land acquired																		600
Acquired land with existing production capacity, percent																			95.00%
Natural gas gathering systems received as part of acquisition																								4
Aggregate principal amount												650,000,000	250,000,000				400,000,000
Revenues	331,582,000	321,790,000	327,785,000	333,966,000	347,341,000	322,237,000	287,829,000	250,096,000	1,315,123,000	1,207,503,000	870,671,000					21,400,000
Net income	(56,547,000)	46,586,000	(19,302,000)	52,439,000	51,661,000	53,360,000	49,819,000	41,027,000	23,176,000	195,867,000	146,484,000					800,000
Asset Divestiture [Abstract]
Percentage of land sold through divestiture														35.00%
Proceeds from divestiture of assets, net of related expenses														$ 226,600,000	$ 44,100,000