Fair Value Measurements (Reconciliations Of Level 3 Fair Value Measurements) (Details) (Collar [Member], USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Collar [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Beginning of period	$ (595)		$ 33,615
Included in earnings	(1,941)	[1]	11,417	[1]
Included in other comprehensive income (loss)	0		2,111
Settlements	0		(11,307)
Transfers out of Level 3 into Level 2	0		(21,924)
End of period	(2,536)		13,912
Total gains for the period included in earnings attributable to the change in unrealized gain relating to assets still held at end of period	$ (1,941)		$ 110

[1] Commodity collars are reported in the Unaudited Condensed Consolidated Statements of Income in oil and natural gas revenues (for cash flow hedges) and loss on derivatives not designated as hedges and hedge ineffectiveness, net, respectively.