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		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Derivative Instruments, Gain Reclassified from Accumulated Other Comprehensive Income into Earnings, Effective Portion			$ 105,600	$ 112,800
Gas Sales [Member] | Costless Collar [Member]
Derivative Instruments, Gain Reclassified from Accumulated Other Comprehensive Income into Earnings, Effective Portion	11,962	18,109	27,060	34,562
Derivative Instruments, Gain (Loss) Recognized in Earnings, Ineffective Portion	(1,083)	(1,045)	1,078	(1,420)
Gas Sales [Member] | Fixed Price Swaps [Member]
Derivative Instruments, Gain Reclassified from Accumulated Other Comprehensive Income into Earnings, Effective Portion	41,736	44,581	78,537	78,254
Derivative Instruments, Gain (Loss) Recognized in Earnings, Ineffective Portion	3,066	(4,139)	999	(3,666)
Costless Collar [Member]
Derivative Instruments, Gain Recognized in Other Comprehensive Income, Effective Portion	14,875	20,215	16,829	55,532
Fixed Price Swaps [Member]
Derivative Instruments, Gain Recognized in Other Comprehensive Income, Effective Portion	$ 120,510	$ 15,220	$ 129,579	$ 90,534