Acquisitions (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended		12 Months Ended					12 Months Ended						12 Months Ended				1 Months Ended					3 Months Ended
	Jun. 30, 2010	Jun. 02, 2010 A Boe	Dec. 31, 2011 Oil [Member] bbl	Dec. 31, 2010 Oil [Member] bbl	Dec. 31, 2009 Oil [Member] bbl	Jun. 02, 2010 Oil [Member] bbl	Dec. 31, 2008 Oil [Member] bbl	Dec. 31, 2011 Natural Gas Liquids [Member] bbl	Dec. 31, 2010 Natural Gas Liquids [Member] bbl	Dec. 31, 2009 Natural Gas Liquids [Member] bbl	Jun. 02, 2010 Natural Gas Liquids [Member]	Dec. 31, 2008 Natural Gas Liquids [Member] bbl	Jun. 02, 2010 Natural Gas Reserves [Member] MMcf	Dec. 31, 2011 Natural Gas [Member] Mcf	Dec. 31, 2010 Natural Gas [Member] Mcf	Dec. 31, 2009 Natural Gas [Member] Mcf	Dec. 31, 2008 Natural Gas [Member] Mcf	Jun. 30, 2010 Southwest Oklahoma And North Texas [Member]	Jul. 20, 2011 Oklahoma And Lipscomb [Member]	Jul. 20, 2011 Oklahoma And Lipscomb [Member] Natural Gas Reserves [Member]	Aug. 31, 2011 Oklahoma Arkoma Woodford And Hartshorne Coal [Member] A	Aug. 31, 2011 Oklahoma Arkoma Woodford And Hartshorne Coal [Member] Natural Gas [Member]	Dec. 31, 2011 South Central Kansas [Member]
Business Acquisition [Line Items]
Payments to acquire oil and gas property	$ 73.7																	$ 17.6	$ 12.3		$ 30.5
Purchase price allocation for proved properties	48.7																		8.4
Undeveloped leasehold	$ 25.0																		$ 3.9				$ 17.3
Gas and oil acreage, developed and undeveloped, net		45,000																			55,000
Operating number of wells		10
Acquired operated wells																			30
Acquired non-operated wells																			59
Operating and non-operating number of wells																					500
Proved developed reserves (BOE)		762,000
Proved developed reserves (volume)			15,618,000	12,773,000	9,183,000	511,000	7,508,000	16,649,000	12,088,000	11,538,000		8,638,000	573	372,311,000	346,928,000	338,217,000	355,824,000
Proved developed reserves (volume) per day, MMcfe																				1.7
Proved developed reserves of NGL, barrels											155,000
Proved developed reserves acquired																				6,600,000,000
Preliminary proved reserves, cubic feet			(2,511,000)	(1,521,000)	(1,286,000)			(2,239,000)	(1,549,000)	(1,488,000)				(44,104,000)	(40,756,000)	(44,063,000)						31,200,000,000
Percentage of proved developed reserves																						83.00%
Net undeveloped acres																		32,000					60,000
Acres acquired held by production																			12,000
Percentage of acre held for production																					96.00%