Natural Gas and Oil Producing Activities (Capitalized Costs Incurred in Natural Gas and Oil Property Acquisition, Exploration and Development Activities) (Details) (USD $)	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Proved property acquisition costs	$ 17,000		$ 0		$ 4,372,000
Unproved property acquisition costs	262,886,000	[1]	229,909,000	[1]	115,217,000	[1]
Exploration costs	63,419,000	[2]	27,062,000	[2]	52,178,000	[2]
Development costs	1,633,784,000		1,524,453,000		1,358,109,000
Capitalized costs incurred	1,960,106,000		1,781,424,000		1,529,876,000
Full cost pool amortization per Mcfe	1.30		1.34		1.51
Capitalized interest based on weighted average cost of borrowings	43,400,000		32,900,000		40,200,000
Acquisition, exploration and development activities	147,700,000		139,200,000		112,900,000
Exploration Program in Canada [Member]
Unproved property acquisition costs	200,000
Exploration costs	$ 2,500,000

[1] (1) Includes $0.2 million and $2.5 million, in 2011 and 2010, respectively, related to our exploration program in New Brunswick, Canada.

[2] (2) Includes $18.4 million and $8.2 million in 2011 and 2010, respectively, related to our exploration program in New Brunswick, Canada.