Fair Value Measurements (Schedule Of Fair Value Of Financial Assets And Liablilities By Level) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset derivative instruments	$ 122,143	$ 105,497
Liability derivative instruments	25,720	34,058
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets from commodity derivative instruments	122,143	105,477
Assets from interest rate derivative instruments		20
Liabilities from commodity derivative instruments	13,484	10,085
Liabilities from interest rate derivative instruments	12,236	23,973
Asset derivative instruments	122,143	105,497
Liability derivative instruments	25,720	34,058
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets from commodity derivative instruments	122,143	105,477
Assets from interest rate derivative instruments		20
Liabilities from commodity derivative instruments	13,484	10,085
Liabilities from interest rate derivative instruments	12,236	23,973
Asset derivative instruments	122,143	105,497
Liability derivative instruments	$ 25,720	$ 34,058