Supplemental Financial Information Exploration and Production (Details 7) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Standardized Measure of Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Beginning Balance	$ 896,100	$ 658,800	$ 969,800
Sales and Transfers of Oil and Gas Produced, Net of Production Costs	(301,500)	(270,000)	(200,900)
Net Increase (Decrease) in Sales and Transfer Prices and Production Costs	82,300	362,400	(364,800)
Extensions Discoveries Less Related Costs	226,300	130,500	70,500
Improved recovery, net of production costs	0	0	0
Increase Due to Purchases of Minerals in Place	9,500	99,800	0
Decrease Due to Sales of Minerals in Place	0	(500)	(1,100)
Changes in Estimated Future Development Costs	51,100	34,100	43,600
Previously Estimated Development Costs Incurred During Period	56,300	43,100	46,400
Accretion of Discount	105,000	76,500	115,900
Changes in Future Income Tax Expense Estimates on Future Cash Flows Related to Proved Oil and Gas Reserves	(55,800)	(103,300)	142,800
Revisions of Previous Quantity Estimates	(92,900)	(132,000)	(155,500)
Standardized Measure of Discounted Future Net Cash Flow of Proved Oil and Gas Reserves, Other	2,400	(3,300)	(7,900)
Standardized Measure of Discounted Future Net Cash Flow of Proved Oil and Gas Reserves, Period Increase (Decrease)	82,700	237,300	(311,000)
Standardized Measure of Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Ending Balance	$ 978,800	$ 896,100	$ 658,800