Acquisitions and Divestitures - Narrative (Details) (USD $)	1 Months Ended		0 Months Ended		0 Months Ended
	Sep. 30, 2012 Bakken [Member]	Sep. 30, 2012 Brazos and Madison Counties [Member]	Sep. 17, 2012 Noble Energy, Inc. [Member] acre systems boe	Jul. 24, 2012 Noble Energy, Inc. [Member]	Sep. 17, 2012 Noble Energy, Inc. [Member] Texas Panhandle [Member] acre locations	Sep. 17, 2012 Noble Energy, Inc. [Member] Western Oklahoma [Member] acre
Business Combination, Description [Abstract]
Land acquired (in acres)			84,000		25,000	59,000
Total payment for the acquisition			$ 594,463,000
Estimated proved reserves of acquired properties (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 debt issued to finance acquisition				400,000,000
Asset Divestitures [Abstract]
Percentage of land sold through divestiture	35.00%
Proceeds from divestiture of assets, net of related expenses	$ 226,600,000	$ 44,100,000