Supplemental Financial Information Exploration and Production Details 2 (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Costs Incurred, Acquisition of Oil and Gas Properties with Proved Reserves	$ 3,999	[1]	$ 89,733	[1]	$ 3,879	[1]
Costs Incurred, Acquisition of Unproved Oil and Gas Properties	63,354	[1]	92,100	[1]	8,771	[1]
Costs Incurred, Exploration Costs	41,775	[1]	33,226	[1]	33,123	[1]
Costs Incurred, Development Costs	161,647	[1]	139,733	[1]	135,202	[1]
Capital Expenditures Related to Oil and Natural Gas Producing Activities	270,775	[1]	354,792	[1]	180,975	[1]
Net change in property, plant and equipment related to future liabilitie for asset retirement obligations that are excluded	$ (1,800)		$ 11,100		$ 2,000

[1] Excludes net additions/(reductions) to property, plant and equipment related to the recognition of future liabilities for asset retirement obligations associated with the plugging and abandonment of natural gas and oil wells, as discussed in Note 10, of $(1.8) million, $11.1 million and $2.0 million for the years ended December 31, 2011, 2010 and 2009, respectively.