Acquisitions and Divestitures Acquisitions and Divestitures (Thompson Field PPA) (Details 1) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2010	Dec. 31, 2012 Thompson Field [Member]	Dec. 31, 2012 Proved Reserves [Member] Thompson Field [Member]	Dec. 31, 2012 Unevaluated Reserves [Domain] Thompson Field [Member]
Business Acquisition [Line Items]
Cash payment	$ 833,900	$ 366,179
Business Acquisition, Purchase Price Allocation, Natural Resources			305,233	12,023
Pipelines and plants		2,000
Other assets		2,957
Asset retirement obligations		(3,306)
Business Acquisition, Purchase Price Allocation, Assets Acquired (Liabilities Assumed), Net		318,907
Goodwill, acquired		$ 47,272