Derivatives and Risk Management (Before Tax Effect of Basis Swaps that do not Qualify for Hedge Accounting Consolidated Statements of Operations) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Basis Swaps [Member]
Derivative [Line Items]
Mark to Market Gain (Loss) on Derivatives Recognized in Earnings	$ (5,279)	$ (1,275)	$ 1,606	$ (270)
Mark to Market Gain (Loss) on Derivatives, Settled Recognized in Earnings	2,020	624	3,115	1,773
Fixed Price Call Options [Member]
Derivative [Line Items]
Mark to Market Gain (Loss) on Derivatives Recognized in Earnings	7,876		(26,852)
Fixed Price Swaps [Member]
Derivative [Line Items]
Mark to Market Gain (Loss) on Derivatives Recognized in Earnings	8,819		96,606
Interest Rate Swaps [Member]
Derivative [Line Items]
Mark to Market Gain (Loss) on Derivatives Recognized in Earnings	(1,312)		1,304
Fair Value Swaps [Member]
Derivative [Line Items]
Mark to Market Gain (Loss) on Derivatives Recognized in Earnings		136		1,162
Mark to Market Gain (Loss) on Derivatives, Settled Recognized in Earnings		$ (5,364)		$ (13,258)