Major Customers And Derivative Counterparties (Schedule Of Derivative Assets And Liabilities By Counterparty) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011
Total derivative assets	$ 482,075
Total derivative liabilities	107,976
Citibank, N.A. [Member]
Total derivative assets	138,267
Total derivative liabilities	6,850
JP Morgan Chase [Member]
Total derivative assets	117,335
Total derivative liabilities	13,070
BNP Paribas [Member]
Total derivative assets	41,879
Total derivative liabilities	6,391
Barclays Capital [Member]
Total derivative assets	35,413
Total derivative liabilities	4,278
Societe Generale [Member]
Total derivative assets	32,376
Total derivative liabilities	2,241
Credit Agricole [Member]
Total derivative assets	28,545
Total derivative liabilities	5,487
Toronto Dominion [Member]
Total derivative assets	20,856
Total derivative liabilities	1,369
Credit Suisse [Member]
Total derivative assets	16,076
Total derivative liabilities	4,779
J. Aron & Company [Member]
Total derivative assets	15,985
Total derivative liabilities	3,139
BMO Financial Group [Member]
Total derivative assets	13,146
Total derivative liabilities	12,365
Wells Fargo Bank, N.A. [Member]
Total derivative assets	12,539
Total derivative liabilities	46,216
Morgan Stanley [Member]
Total derivative assets	4,923
Total derivative liabilities	774
Den Norske Bank [Member]
Total derivative assets	4,582
Total derivative liabilities
Merrill Lynch [Member]
Total derivative assets	153
Total derivative liabilities	$ 1,017