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	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Fixed Price Swaps [Member]
Derivative [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income, Effective Portion	$ 178,660	$ 714,740
Costless Collar [Member]
Derivative [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income, Effective Portion	39,247	144,126
Gas Sales [Member] | Fixed Price Swaps [Member]
Derivative [Line Items]
Derivative Instruments, Gain Reclassified from Accumulated Other Comprehensive Income into Earnings, Effective Portion	413,410	256,229
Derivative Instruments, Gain (Loss) Recognized in Earnings, Ineffective Portion	2,450	(4,018)
Gas Sales [Member] | Costless Collar [Member]
Derivative [Line Items]
Derivative Instruments, Gain Reclassified from Accumulated Other Comprehensive Income into Earnings, Effective Portion	218,119	65,047
Derivative Instruments, Gain (Loss) Recognized in Earnings, Ineffective Portion	$ (24)	$ (137)