Derivatives and Risk Management (Before Tax Effect of Derivatives on Consolidated Financial Statements) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Fixed Price Swaps [Member]
Derivative Instruments, Gain Recognized in Other Comprehensive Income, Effective Portion	$ 714,740	$ 166,722
Derivative Instruments, Gain Reclassified from Accumulated Other Comprehensive Income into Earnings, Effective Portion	256,229	230,707
Derivative Instruments, Gain (Loss) Recognized in Earnings, Ineffective Portion	(4,018)	(4,769)
Costless Collar [Member]
Derivative Instruments, Gain Recognized in Other Comprehensive Income, Effective Portion	144,126	132,438
Derivative Instruments, Gain Reclassified from Accumulated Other Comprehensive Income into Earnings, Effective Portion	65,047	70,775
Derivative Instruments, Gain (Loss) Recognized in Earnings, Ineffective Portion	(137)	(1,999)
Gas Sales [Member]
Derivative Instruments, Unrealized Gain (Loss) Recognized in Earnings	5,222	11,434
Derivative Instruments, Realized Gain (Loss) Recognized in Earnings	$ (2,135)	$ (12,098)