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		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Fixed Price Swaps [Member]
Derivative Instruments, Gain Recognized in Other Comprehensive Income, Effective Portion	$ (47,829)	$ 120,510	$ 171,128	$ 129,579
Costless Collar [Member]
Derivative Instruments, Gain Recognized in Other Comprehensive Income, Effective Portion	(12,370)	14,875	44,141	16,829
Gas Sales [Member] | Fixed Price Swaps [Member]
Derivative Instruments, Gain Reclassified from Accumulated Other Comprehensive Income into Earnings, Effective Portion	128,894	41,736	235,205	78,537
Derivative Instruments, Gain (Loss) Recognized in Earnings, Ineffective Portion	(2,303)	3,066	1,996	999
Gas Sales [Member] | Costless Collar [Member]
Derivative Instruments, Gain Reclassified from Accumulated Other Comprehensive Income into Earnings, Effective Portion	65,732	11,962	121,042	27,060
Derivative Instruments, Gain (Loss) Recognized in Earnings, Ineffective Portion	$ (371)	$ (1,083)	$ 540	$ 1,078