Natural Gas And Oil Producing Activities (Unaudited) (Capitalized Costs Incurred in Natural Gas and Oil Property Acquisition, Exploration and Development Activities) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Proved property acquisition costs	$ 572				$ 17
Unproved property acquisition costs	168,404	[1]	220,822	[1]	262,886	[1]
Exploration costs	192,164	[2]	197,280	[2]	63,419	[2]
Development costs	1,662,138		1,492,841		1,633,784
Capitalized costs incurred	2,023,278		1,910,943		1,960,106
Full cost pool amortization per Mcfe	1.08		1.31		1.30
Costs Incurred in Natural Gas and Oil Exploration and Development [Member] | Exploration Program in Canada [Member]
Unproved property acquisition costs	17,100		3,600		200
Exploration costs	$ 11,500		$ 2,500		$ 18,400

[1] Includes $17.1 million, $3.6 million and $0.2 million, in 2013, 2012 and 2011, respectively, related to our exploration program in Canada.

[2] Includes $11.5 million, $2.5 million and $18.4 million in 2013, 2012 and 2011, respectively, related to our exploration program in Canada.