Acquisitions and Divestitures Acquisitions and Divestitures (Bakken PPA) (Details 1) (USD $) In Thousands, unless otherwise specified	10 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Business Acquisition [Line Items]
Proved	$ 8,314,196	$ 6,963,211
Unevaluated	1,177,564	809,154
CO2 properties	1,094,699	1,032,653
Asset retirement obligations	(123,994)	(102,730)
Bakken Exchange Transaction [Member]
Business Acquisition [Line Items]
Fair value of net assets transferred	1,866,107
Cash	1,277,041	1,300,000
Proved	182,289
Unevaluated	90,690
CO2 properties	314,505
Other property and equipment	23,424
Other assets	477
Other liabilities	(8,528)
Asset retirement obligations	(13,791)
Business Acquisition, Purchase Price Allocation, Assets Acquired Less (Liabilities Assumed)	$ 1,866,107