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Fair Value (Fair Value Of Assets And Liabilities Measured On Recurring Basis) (Details) - USD ($)
$ in Thousands
Jun. 30, 2017
Dec. 31, 2016
Jun. 30, 2016
Dec. 31, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Gross Asset $ 46,766 $ 61,191    
Liability 144,548 150,381    
Cash and cash equivalents 13,410 8,507 $ 13,522 $ 10,484
Fixed Income Securities [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Cash and cash equivalents 200 400    
Fair Value, Measurements, Recurring [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Counterparty and collateral netting, assets (36,973) (50,546)    
Interest rate swaps 9,449 8,750    
Total 17,576 17,915    
Counterparty and collateral netting, liabilities (88,310) (95,304)    
Interest rate swaps 36,843 34,730    
Total 56,238 55,077    
Fair Value, Measurements, Recurring [Member] | Natural Gas Exchange Agreements [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Counterparty and collateral netting, assets (79) (69)    
Derivative Asset 0 0    
Counterparty and collateral netting, liabilities (79) (69)    
Derivative Liability 4,173 5,885    
Fair Value, Measurements, Recurring [Member] | Power Exchange Agreements [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Counterparty and collateral netting, assets   (25)    
Derivative Asset   0    
Counterparty and collateral netting, liabilities 0 (25)    
Derivative Liability 13,784 13,449    
Fair Value, Measurements, Recurring [Member] | Interest Rate Swap [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Counterparty and collateral netting, assets (1,853) (4,348)    
Counterparty and collateral netting, liabilities (43,423) (39,248)    
Fair Value, Measurements, Recurring [Member] | Energy commodity derivatives [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Counterparty and collateral netting, assets (35,041) (46,099)    
Derivative Asset 157 1,895    
Counterparty and collateral netting, liabilities (44,808) (55,957)    
Derivative Liability 1,395 914    
Fair Value, Measurements, Recurring [Member] | Power Option Agreement [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Counterparty and collateral netting, liabilities 0 0    
Derivative Liability 43 76    
Fair Value, Measurements, Recurring [Member] | Foreign Exchange Contract [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Counterparty and collateral netting, assets 0 (5)    
Derivative Asset 187 0    
Counterparty and collateral netting, liabilities   (5)    
Derivative Liability   23    
Fair Value, Measurements, Recurring [Member] | Fixed Income Securities [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Deferred compensation assets: 1,716 1,789    
Fair Value, Measurements, Recurring [Member] | Equity Securities [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Deferred compensation assets: 6,067 5,481    
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Total 7,783 7,270    
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: 1,716 1,789    
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Equity Securities [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Deferred compensation assets: 6,067 5,481    
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Interest rate swaps 11,302 13,098    
Total 46,687 61,097    
Interest rate swaps 80,266 73,978    
Total 126,469 130,877    
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Energy commodity derivatives [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Gross Asset 35,198 47,994    
Liability 46,203 56,871    
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Foreign Exchange Contract [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Gross Asset 187 5    
Liability   28    
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Total 79 94    
Total 18,079 19,504    
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Natural Gas Exchange Agreements [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Gross Asset 79 69    
Liability 4,252 5,954    
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Power Exchange Agreements [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Gross Asset   25    
Liability 13,784 13,474    
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Power Option Agreement [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Liability $ 43 $ 76