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		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Basis Swaps [Member]
Derivative [Line Items]
Unrealized Gain (Loss) Recognized in Earnings	$ 9,835	$ (218)	$ 6,885	$ 1,005
Realized Gain Recognized in Earnings	88	131	1,095	1,149
Fixed Price Call Options [Member]
Derivative [Line Items]
Unrealized Gain (Loss) Recognized in Earnings	22,354		(34,728)
Fixed Price Swaps [Member]
Derivative [Line Items]
Unrealized Gain (Loss) Recognized in Earnings	58,556		87,787
Interest Rate Swaps [Member]
Derivative [Line Items]
Unrealized Gain (Loss) Recognized in Earnings	$ 2,616		$ 2,616