Derivatives and Risk Management (Before Tax Effect of all Cash Flow Hedges on the Condensed Consolidated Financial Statements) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Fixed Price Swaps [Member]
Derivative [Line Items]
Gain (Loss) Recognized in Other Comprehensive Income (Effective Portion)	$ 12,515	$ (55,039)	$ 51,234	$ 116,089
Costless Collar [Member]
Derivative [Line Items]
Gain (Loss) Recognized in Other Comprehensive Income (Effective Portion)		(3,497)		40,644
Gas Sales [Member] | Fixed Price Swaps [Member]
Derivative [Line Items]
Gain Reclassified from Accumulated Other Comprehensive Income into Earnings (Effective Portion)	93,280	102,789	218,236	337,994
Gain (Loss) Recognized in Earnings (Ineffective Portion)	(1,452)	(165)	(2,183)	1,831
Gas Sales [Member] | Costless Collar [Member]
Derivative [Line Items]
Gain Reclassified from Accumulated Other Comprehensive Income into Earnings (Effective Portion)		54,489		175,531
Gain (Loss) Recognized in Earnings (Ineffective Portion)		$ (373)		$ 167