Natural Gas and Oil Producing Activities (Capitalized Costs of Gas and Oil Properties and the Related Accumulated Depreciation, Depletion and Amortization) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011		Dec. 31, 2010
Natural Gas And Oil Producing Activities Abstract
Proved properties	$ 8,601,818		$ 7,037,746
Unproved properties	942,890	[1]	712,117	[1]
Total capitalized costs	9,544,708		7,749,863
Less: Accumulated depreciation, depletion and amortization	4,092,410		3,444,477
Net capitalized costs	$ 5,452,298		$ 4,305,386

[1] (1) Includes $27.9 and $10.7 million related to our exploration program in New Brunswick, Canada at December 31, 2011 and 2010, respectively.