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Fair Value (Fair Value Of Assets And Liabilities Measured On Recurring Basis) (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Sep. 30, 2017
Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Gross Asset $ 55,941 $ 51,506    
Liability 71,127 161,467    
Cash and cash equivalents 21,170 16,172 $ 14,716 $ 8,507
Fixed Income Securities [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Cash and cash equivalents 400 200    
Fair Value, Measurements, Recurring [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Counterparty and collateral netting, assets (37,002) (45,308)    
Interest rate swaps 18,029 4,903    
Total 27,470 14,467    
Counterparty and collateral netting, liabilities (56,351) (107,716)    
Interest rate swaps 4,448 35,969    
Total 14,776 53,751    
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 (22) (183)    
Derivative Asset 0 0    
Counterparty and collateral netting, liabilities (22) (183)    
Derivative Liability 3,004 3,164    
Fair Value, Measurements, Recurring [Member] | Power Exchange Agreements [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Counterparty and collateral netting, liabilities 0 0    
Derivative Liability 5,909 13,245    
Fair Value, Measurements, Recurring [Member] | Interest Rate Swap [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Counterparty and collateral netting, assets (129) (2,574)    
Counterparty and collateral netting, liabilities (129) (37,544)    
Fair Value, Measurements, Recurring [Member] | Energy commodity derivatives        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Counterparty and collateral netting, assets (36,851) (42,550)    
Derivative Asset 884 1,264    
Counterparty and collateral netting, liabilities (56,200) (69,988)    
Derivative Liability 1,415 1,354    
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    
Derivative Liability   19    
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 (1)    
Derivative Asset 26 31    
Counterparty and collateral netting, liabilities   (1)    
Derivative Liability   0    
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Total 8,531 8,269    
Total 0 0    
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Interest rate swaps 18,158 7,477    
Total 55,919 51,323    
Interest rate swaps 4,577 73,513    
Total 62,192 144,856    
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Energy commodity derivatives        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Gross Asset 37,735 43,814    
Liability 57,615 71,342    
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Foreign Exchange Contract [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Gross Asset 26 32    
Liability   1    
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Total 22 183    
Total 8,935 16,611    
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 22 183    
Liability 3,026 3,347    
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Power Exchange Agreements [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Liability 5,909 13,245    
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Power Option Agreement [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Liability   19    
Fixed Income Funds [Member] | Fair Value, Measurements, Recurring [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Deferred compensation assets: 1,819 1,638    
Fixed Income Funds [Member] | Fair Value, Measurements, Recurring [Member] | Level 1 [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Deferred compensation assets: 1,819 1,638    
Equity Funds [Member] | Fair Value, Measurements, Recurring [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Deferred compensation assets: 6,712 6,631    
Equity Funds [Member] | Fair Value, Measurements, Recurring [Member] | Level 1 [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Deferred compensation assets: $ 6,712 $ 6,631