Fair Value Measurements (Reconciliations Of Level 3 Fair Value Measurements) (Details) (USD $) In Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2011		Sep. 30, 2010		Sep. 30, 2011		Sep. 30, 2010
Commodity Swap [Member]
Beginning of period	$ 11,749				$ 10,868
Included in earnings (loss)	4,046	[1]			11,923	[1]
Included in other comprehensive income (loss)	13,182				13,264
Purchases, issuance and settlements	(3,848)				(10,926)
End of period	25,129				25,129
Total gains for the period included in earnings attributable to the change in unrealized gains relating to assets still held at end of period	198				997
Commodity Swaps And Collars [Member]
Beginning of period			33,313				19,948
Included in earnings (loss)			16,558	[2]			44,322	[2]
Included in other comprehensive income (loss)			(1,266)				10,646
Purchases, issuance and settlements			(16,809)				(43,120)
End of period			31,796				31,796
Total gains for the period included in earnings attributable to the change in unrealized gains relating to assets still held at end of period			(251)				1,202
Interest Rate Swaps [Member]
Beginning of period	0		(1,971)		(1,614)		(1,948)
Included in earnings (loss)	0	[1]	(298)	[2]	(1,734)	[1]	(907)	[2]
Included in other comprehensive income (loss)	0		39		1,614		16
Purchases, issuance and settlements	0		298		1,734		907
End of period	0		(1,932)		0		(1,932)
Total gains for the period included in earnings attributable to the change in unrealized gains relating to assets still held at end of period	$ 0		$ 0		$ 0		$ 0

[1] Interest rate swaps and commodity swaps are reported in the unaudited condensed consolidated statements of income in interest, net and revenues, respectively.

[2] Interest rate swaps and commodity swaps and collars are reported in the unaudited condensed consolidated statements of income in interest, net and revenues, respectively.