Natural Gas and Oil Producing Activities (Unaudited) (Composition of Net Unevaluated Costs Excluded from Amortization (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended			72 Months Ended	108 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2012
Property aquisition costs	$ 193,258	$ 232,615	$ 123,736	$ 45,081	$ 594,690	[1]
Exploration and Development Costs	233,888	65,479	15,357	36,958	351,682	[1]
Capitalized interest	11,578	25,163	18,989	21,786	77,516	[1]
Net unevaluated costs excluded from amortization	438,724	323,257	158,082	103,825	1,023,888
Wells In Progress [Member]
Net unevaluated costs excluded from amortization	270,800
Undeveloped Properties New Ventures - Excluding Canada [Member]
Net unevaluated costs excluded from amortization	387,100
Undeveloped Properties Marcellus Shale Play [Member]
Net unevaluated costs excluded from amortization	145,600
Undeveloped Properties Fayetteville Shale Play [Member]
Net unevaluated costs excluded from amortization	44,000
Seismic Costs Fayetteville Shale Play [Member]
Net unevaluated costs excluded from amortization	25,900
Exploration Program in Canada [Member]
Net unevaluated costs excluded from amortization	40,400
Composition of Net Unevaluated Costs [Member] | Exploration Program in Canada [Member]
Property aquisition costs		14,800
Exploration and Development Costs		22,500
Capitalized interest		$ 3,100

[1] Property acquisition costs include $14.8 million, exploration costs include $22.5 million and capitalized interest includes $3.1 million related to our exploration program in Canada.