Fair Value Of Assets And Liabilities (Schedule Of Estimated Fair Value Of Long-Term Obligations Included In Financial Statements) (Details) $ in Thousands	Dec. 31, 2016 USD ($)
Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term obligations (including current installments)	$ 470,443
Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term obligations (including current installments)	481,220
2011 Series A Bonds [Member] | Level 2 [Member] | Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term obligations (including current installments)	221,667
2011 Series A Bonds [Member] | Level 2 [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term obligations (including current installments)	231,057
2012 Series A Bonds [Member] | Level 2 [Member] | Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term obligations (including current installments)	205,000
2012 Series A Bonds [Member] | Level 2 [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term obligations (including current installments)	209,242
2016 CoBank Note [Member] | Level 2 [Member] | Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term obligations (including current installments)	43,776
2016 CoBank Note [Member] | Level 2 [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term obligations (including current installments)	$ 40,921