Fair Value Measurements (Schedule Of Fair Value Of Financial Assets And Liablilities By Level) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset derivative instruments	$ 98,500	$ 122,143
Liability derivative instruments	26,366	25,720
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets from commodity derivative instruments	98,500	122,143
Liabilities from commodity derivative instruments	15,157	13,484
Liabilities from interest rate derivative instruments	11,209	12,236
Asset derivative instruments	98,500	122,143
Liability derivative instruments	26,366	25,720
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	98,500	122,143
Liabilities from commodity derivative instruments	15,157	13,484
Liabilities from interest rate derivative instruments	11,209	12,236
Asset derivative instruments	98,500	122,143
Liability derivative instruments	$ 26,366	$ 25,720