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	Dec. 31, 2012		Dec. 31, 2011
Natural Gas And Oil Properties [Abstract]
Proved properties	$ 10,259,226		$ 8,601,818
Unproved properties	1,023,888	[1]	942,890	[1]
Total capitalized costs	11,283,114		9,544,708
Less: Accumulated depreciation, depletion and amortization	6,774,174		4,092,410
Net capitalized costs	$ 4,508,940		$ 5,452,298

[1] Includes $40.4 and $27.9 million related to our exploration program in Canada as of December 31, 2012 and 2011, respectively.