Derivatives and Risk Management (Before Tax Effect of all Cash Flow Hedges on the Condensed Consolidated Financial Statements) (Details) (USD $) In Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Derivative Instruments, Gain Reclassified from Accumulated Other Comprehensive Income into Earnings, Effective Portion			$ 186,600	$ 194,600
Gas Sales [Member] | Costless Collar [Member]
Derivative Instruments, Gain Reclassified from Accumulated Other Comprehensive Income into Earnings, Effective Portion	13,918	16,073	40,978	50,635
Derivative Instruments, Gain (Loss) Recognized in Earnings, Ineffective Portion	(826)	(157)	252	(1,577)
Gas Sales [Member] | Fixed Price Swaps [Member]
Derivative Instruments, Gain Reclassified from Accumulated Other Comprehensive Income into Earnings, Effective Portion	67,125	65,761	145,662	144,015
Derivative Instruments, Gain (Loss) Recognized in Earnings, Ineffective Portion	(1,754)	(258)	(755)	(3,924)
Costless Collar [Member]
Derivative Instruments, Gain Recognized in Other Comprehensive Income, Effective Portion	48,315	84,750	65,144	140,282
Fixed Price Swaps [Member]
Derivative Instruments, Gain Recognized in Other Comprehensive Income, Effective Portion	$ 230,783	$ 75,864	$ 360,362	$ 166,398