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Fair Value (Fair Value Of Assets And Liabilities Measured On Recurring Basis) (Details) - USD ($)
$ in Thousands
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Asset $ 76,865 $ 99,045    
Liability 210,512 229,790    
Cash and cash equivalents 10,484 22,143 $ 82,574 $ 75,464
Fixed Income Securities [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Cash and cash equivalents 600 800    
Fair Value, Measurements, Recurring [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset (74,634) (97,060)    
Interest rate swaps 1,548 460    
Assets Held-in-trust   1,600    
Total 9,719 11,452    
Interest Rate Derivative Liabilities, at Fair Value 85,498 48,182    
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset 89,160 141,450    
Total 121,352 88,340    
Fair Value, Measurements, Recurring [Member] | Energy commodity derivatives [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset (73,954) (95,204)    
Derivative Asset 683 1,525    
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset 88,480 110,714    
Derivative Liability 8,713 16,380    
Fair Value, Measurements, Recurring [Member] | Natural Gas Exchange Agreements [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset (678) (1,349)    
Derivative Asset 0 0    
Liability 5,039      
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset 678 1,349    
Derivative Liability 5,039 35    
Fair Value, Measurements, Recurring [Member] | Power Exchange Agreements [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Liability 21,961      
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset 0 0    
Derivative Liability 21,961 23,299    
Fair Value, Measurements, Recurring [Member] | Power Option Agreement [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Liability 124      
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset 0      
Derivative Liability 124      
Fair Value, Measurements, Recurring [Member] | Commodity Option [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset   0    
Derivative Liability   424    
Fair Value, Measurements, Recurring [Member] | Interest Rate Swap [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset 0 (506)    
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset 0 29,386    
Fair Value, Measurements, Recurring [Member] | Foreign Exchange Contract [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset (2) (1)    
Derivative Asset 0 0    
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset 2 1    
Derivative Liability 17 20    
Fair Value, Measurements, Recurring [Member] | Fixed Income Securities [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Deferred compensation assets: 1,727 1,793    
Fair Value, Measurements, Recurring [Member] | Equity Securities [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Deferred compensation assets: 5,761 6,074    
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Assets Held-in-trust   1,600    
Total 7,488 9,467    
Total 0 0    
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Fixed Income Securities [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Deferred compensation assets: 1,727 1,793    
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Equity Securities [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Deferred compensation assets: 5,761 6,074    
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Interest rate swaps 1,548 966    
Total 76,187 97,696    
Interest Rate Derivative Liabilities, at Fair Value 85,498 77,568    
Total 182,710 204,683    
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Energy commodity derivatives [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Asset 74,637 96,729    
Liability 97,193 127,094    
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Foreign Exchange Contract [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Asset 2 1    
Liability 19 21    
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Total 678 1,349    
Total 27,802 25,107    
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Natural Gas Exchange Agreements [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Asset 678 1,349    
Liability 5,717 1,384    
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Power Exchange Agreements [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Liability 21,961 23,299    
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Power Option Agreement [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Liability $ 124      
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Commodity Option [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Liability   $ 424