Fair Value Measurements (Reconciliations Of Level 3 Fair Value Measurements) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Beginning of period		$ (7,122)	$ 9,094
Included in earnings	[1]	7,791	(9,042)
Settlements		(875)	(7,174)
End of period		(206)	(7,122)
Total gains (losses) for the period included in earnings attributable to the change in unrealized loss relating to assets still held at end of period		$ 6,916	$ (16,216)

[1] Commodity derivatives are reported in the Consolidated Statements of Operations in gain (loss) on derivatives.