Natural Gas and Oil Producing Activities (Unaudited) (Capitalized Costs Incurred in Natural Gas and Oil Property Acquisition, Exploration and Development Activities) (Details) (USD $)	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Proved property acquisition costs			$ 17,000
Unproved property acquisition costs	220,822,000	[1]	262,886,000	[1]	229,909,000	[1]
Exploration costs	197,280,000	[2]	63,419,000	[2]	27,062,000	[2]
Development costs	1,492,841,000		1,633,784,000		1,524,453,000
Capitalized costs incurred	1,910,943,000		1,960,106,000		1,781,424,000
Full cost pool amortization per Mcfe	1.31		1.30		1.34
Capitalized interest based on weighted average cost of borrowings	62,100,000		43,400,000		32,900,000
Acquisition, exploration and development activities	159,700,000		147,700,000		139,200,000
Exploration Program in Canada [Member]
Unproved property acquisition costs					2,500,000
Exploration costs					8,200,000
Costs Incurred in Natural Gas and Oil Exploration and Development [Member] | Exploration Program in Canada [Member]
Unproved property acquisition costs	3,600,000		200,000
Exploration costs	2,500,000		18,400,000
Capitalized Costs [Member] | Exploration Program in Canada [Member]
Unproved property acquisition costs	$ 40,400,000		$ 27,900,000

[1] Includes $3.6 million, $0.2 million and $2.5 million, in 2012, 2011 and 2010, respectively, related to our exploration program in Canada.

[2] Includes $2.5 million, $18.4 million and $8.2 million in 2012, 2011 and 2010, respectively, related to our exploration program in Canada.