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, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Fixed Price Swaps [Member]
Derivative [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income, Effective Portion	$ (55,039)	$ 230,783	$ 116,089	$ 360,362
Costless Collar [Member]
Derivative [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income, Effective Portion	(3,497)	48,315	40,644	65,144
Gas Sales [Member] | Fixed Price Swaps [Member]
Derivative [Line Items]
Derivative Instruments, Gain Reclassified from Accumulated Other Comprehensive Income into Earnings, Effective Portion	102,789	67,125	337,994	145,662
Derivative Instruments, Gain (Loss) Recognized in Earnings, Ineffective Portion	(165)	(1,754)	1,831	(755)
Gas Sales [Member] | Costless Collar [Member]
Derivative [Line Items]
Derivative Instruments, Gain Reclassified from Accumulated Other Comprehensive Income into Earnings, Effective Portion	54,489	13,918	175,531	40,978
Derivative Instruments, Gain (Loss) Recognized in Earnings, Ineffective Portion	$ (373)	$ (826)	$ 167	$ 252