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
	Mar. 31, 2012	Mar. 31, 2011
Costless Collar [Member]
Derivative Instruments, Gain Recognized in Other Comprehensive Income, Effective Portion	$ 56,511	$ 1,954
Fixed Price Swaps [Member]
Derivative Instruments, Gain Recognized in Other Comprehensive Income, Effective Portion	218,957	9,069
Gas Sales [Member] | Costless Collar [Member]
Derivative Instruments, Gain Reclassified from Accumulated Other Comprehensive Income into Earnings, Effective Portion	55,310	15,098
Derivative Instruments, Gain (Loss) Recognized in Earnings, Ineffective Portion	911	2,161
Gas Sales [Member] | Fixed Price Swaps [Member]
Derivative Instruments, Gain Reclassified from Accumulated Other Comprehensive Income into Earnings, Effective Portion	106,311	36,801
Derivative Instruments, Gain (Loss) Recognized in Earnings, Ineffective Portion	$ 4,299	$ (2,067)