Derivative Financial Instruments (Schedule Of Marketing And Basis Transfer Derivative Contracts) (Details) (Two Thousand And Thirteen Index Basis Swap Contracts [Member], Basic Index Swaps [Member])	Mar. 31, 2013 MMBTU
Two Thousand And Thirteen Index Basis Swap Contracts [Member] | Basic Index Swaps [Member]
Derivative [Line Items]
Average Daily Index Swaps Volume	4,364
Price Differential	0.34