Acquisitions and Divestitures (Details Textuals) (USD $)	9 Months Ended		1 Months Ended		9 Months Ended		1 Months Ended		3 Months Ended		3 Months Ended	6 Months Ended		1 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Jun. 30, 2012 Thompson Field [Member] Rate	Feb. 29, 2012 Non-core Gulf Coast Assets [Member]	Sep. 30, 2012 Non-core Gulf Coast Assets [Member]	Sep. 30, 2012 Paradox Basin [Member]	Apr. 30, 2012 Paradox Basin [Member] Scenario, Plan [Member]	Apr. 30, 2012 Paradox Basin [Member] Scenario, Actual [Member]	Dec. 31, 2012 Bakken Exchange Transaction [Member] Rate	Sep. 30, 2013 Bakken Exchange Transaction [Member]	Sep. 30, 2013 Cedar Creek Anticline [Member]	Sep. 30, 2013 Cedar Creek Anticline [Member]		Jan. 31, 2013 Cedar Creek Anticline [Member] Scenario, Plan [Member]
Business Acquisition [Line Items]
Business Acquisition, Cost of Acquired Entity, Cash Paid			$ 366,200,000									$ 1,001,707,000	[1]	$ 1,050,000,000
Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual											97,100,000	189,800,000
Business Combination, Pro Forma Information, Earnings or Loss of Acquiree since Acquisition Date, Actual											70,900,000	138,800,000
Business Acquisition, Purchase Price Allocation, Current Assets, Cash and Cash Equivalents									1,300,000,000	1,277,041,000
Percent of Overriding Royalty Interest in CO2 Properties Acquired In A Business Acquisition									33.33%
Net working interest acquired in purchase of oil and natural gas properties			100.00%
Net revenue interest acquired in purchase of oil and natural gas properties			84.70%
Net Revenue Interest Retained By Seller			5.00%
Oil Production Threshold (in Bbls/d)			3,000
Proceeds from Sale of Oil and Gas Property and Equipment	6,312,000	33,973,000		141,800,000			75,000,000	68,500,000
Gain (Loss) on Sale of Property					$ 0	$ 0

[1] $989.0 million of this cash consideration was paid through a qualified intermediary from cash placed in qualifying trust accounts from a portion of the proceeds received from the Bakken Exchange Transaction (as defined below) in order to enable a like-kind-exchange transaction for federal income tax purposes. As such, this amount is not reflected as a cash payment to purchase oil and natural gas properties in our Unaudited Condensed Consolidated Statement of Cash Flows.