Natural Gas And Oil Producing Activities (Unaudited) (Capitalized Costs of Gas and Oil Properties and the Related Accumulated Depreciation, Depletion, and Amortization) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Proved properties	$ 12,337,372		$ 10,259,226
Unproved properties	956,469	[1]	1,023,888	[1]
Total capitalized costs	13,293,841		11,283,114
Less: Accumulated depreciation, depletion and amortization	7,481,335		6,774,174
Net capitalized costs	5,812,506		4,508,940
Unproved property acquisition costs	168,404	[2]	220,822	[2]	262,886	[2]
Capitalized Costs [Member] | Exploration Program in Canada [Member]
Unproved property acquisition costs	$ 72,300		$ 40,400

[1] Includes $72.3 and $40.4 million related to our exploration program in Canada as of December 31, 2013 and 2012, respectively.

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