Supplemental Information about Oil & Gas Producing Activities (Unaudited) (Costs Incurred in Oil and Gas Property Acquisition, Exploration, and Development Activities) (Details) (USD $)	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Costs Incurred, Acquisition of Oil and Natural Gas Properties
Proved properties	$ 149,158,000		$ 334,409,000		$ 13,497,000
Unproved properties	6,632,000		0		0
Development	544,114,000		320,927,000		138,168,000
Exploration	627,000		2,310,000		209,000
Total	700,531,000	[1]	657,646,000	[1]	151,874,000	[1]
Capitalized interest incurred	$ 29,100,000		$ 28,300,000		$ 30,100,000

[1] The total above does not reflect $29.1 million, $28.3 million, and $30.1 million of capitalized interest incurred in 2011, 2010, and 2009, respectively.