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, 2013	Mar. 31, 2012
Fixed Price Swaps [Member]
Derivative [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income, Effective Portion	$ (73,902)	$ 218,957
Costless Collar [Member]
Derivative [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income, Effective Portion		56,511
Gas Sales [Member] | Fixed Price Swaps [Member]
Derivative [Line Items]
Derivative Instruments, Gain Reclassified from Accumulated Other Comprehensive Income into Earnings, Effective Portion	78,621	106,311
Derivative Instruments, Gain (Loss) Recognized in Earnings, Ineffective Portion	1,301	4,299
Gas Sales [Member] | Costless Collar [Member]
Derivative [Line Items]
Derivative Instruments, Gain Reclassified from Accumulated Other Comprehensive Income into Earnings, Effective Portion		55,310
Derivative Instruments, Gain (Loss) Recognized in Earnings, Ineffective Portion		$ 911