Acquisitions and Divestitures (Earnings from Discontinued Operations) (Details) (USD $)	12 Months Ended			3 Months Ended	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Mar. 31, 2009 Denver Julesburg	Dec. 31, 2009 Denver Julesburg
Divestitures
Impairment of Oil and Gas Properties from discontinued operations, before tax					$ 9,600,000
Sales of oil and natural gas					11,555,000	[1]
Loss on sale of asset					(908,000)
Other revenue					623,000
Total revenues					11,270,000
Realized and unrealized (gain) on derivatives, net					(13,786,000)
Other expenses					15,799,000	[2]
Total expenses					2,013,000
Earnings from discontinued operations, before income taxes					9,257,000
Provision for income taxes					2,451,000
Earnings from discontinued operations, net of taxes	0	0	6,806,000		6,806,000
Realized gain included in Sales of oil and natural gas reclassified to discontinued operations					6,200,000
Amount of interest to discontinued operations					800,000
Reclassified gain from AOCL to the Statements of Operations					14,300,000
Unrealized net loss on hedges on Statement of Operations				$ (500,000)

[1] A $6.2 million realized gain included in sales of oil and natural gas was reclassified to discontinued operations for the year ended December 31, 2009.

[2] Includes $9.6 million of impairment charges related to the sale of the Company's assets in the DJ and $0.8 million of interest allocated to discontinued operations based on the ratio of net assets to the sum of total net assets