Derivative Activities (Fair Value Of Derivative Contracts) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Current asset derivatives	$ 29,762	$ 45,522
Noncurrent asset derivatives	68,738	76,621
Asset derivative instruments	98,500	122,143
Current liability derivatives	12,254	8,727
Noncurrent liability derivatives	14,112	16,993
Liability derivative instruments	26,366	25,720
Commodity Contract [Member]
Derivatives, Fair Value [Line Items]
Current asset derivatives	29,762	45,522
Noncurrent asset derivatives	68,738	76,621
Asset derivative instruments	98,500	122,143
Current liability derivatives	7,676	4,130
Noncurrent liability derivatives	7,481	9,354
Liability derivative instruments	15,157	13,484
Interest Rate Contract [Member]
Derivatives, Fair Value [Line Items]
Current liability derivatives	4,578	4,597
Noncurrent liability derivatives	6,631	7,639
Liability derivative instruments	$ 11,209	$ 12,236