Acquisitions and Divestitures Acquisitions and Divestitures (Bakken Exchange Transaction PPA) (Details) (USD $)	12 Months Ended	6 Months Ended
	Dec. 31, 2012	Dec. 21, 2012 Bakken Exchange Transaction [Member]	Dec. 31, 2012 Bakken Exchange Transaction [Member]	Dec. 31, 2012 Proved Reserves [Member] Bakken Exchange Transaction [Member]	Dec. 31, 2012 Unevaluated Reserves [Domain] Bakken Exchange Transaction [Member]
Business Acquisition [Line Items]
Fair value of net assets transferred			$ 1,903,280,000
Cash			1,331,684,000
Business Acquisition, Purchase Price Allocation, Natural Resources				201,301,000	98,635,000
CO2 properties			314,505,000
Other assets			477,000
Other liabilities			(29,531,000)
Asset retirement obligation			(13,791,000)
Fair value of net assets acquired			1,903,280,000
Operating revenues Net Of Capital And Lease Operating Expense Between Effective Date And Closing Date Of Transaction		41,700,000
Anticipated Pre Adjusted Cash Paid For Purchase Of Oil And Natural Gas Properties	$ 1,050,000,000