Derivatives and Risk Management (Before Tax Effect of Basis Swaps that do not Qualify for Hedge Accounting Consolidated Statements of Operations) (Details) (Gas Sales [Member], USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Basis Swaps [Member]
Derivative [Line Items]
Derivative Instruments, Unrealized Gain (Loss) Recognized in Earnings	$ (2,950)	$ 1,223
Derivative Instruments, Realized Gain (Loss) Recognized in Earnings	1,007	1,018
Fixed Price Call Options [Member]
Derivative [Line Items]
Derivative Instruments, Unrealized Gain (Loss) Recognized in Earnings	(57,082)
Fixed Price Swaps [Member]
Derivative [Line Items]
Derivative Instruments, Unrealized Gain (Loss) Recognized in Earnings	$ 29,232