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Fair Value (Fair Value Of Assets And Liabilities Measured On Recurring Basis) (Details) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Jun. 30, 2017
Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Gross Asset $ 55,291 $ 51,506    
Liability 81,500 161,467    
Cash and cash equivalents 35,333 16,172 $ 13,410 $ 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 (42,866) (45,308)    
Interest rate swaps 12,314 4,903    
Total 20,763 14,467    
Counterparty and collateral netting, liabilities (65,237) (107,716)    
Interest rate swaps 4,901 35,969    
Total 16,263 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 (25) (183)    
Derivative Asset 0 0    
Counterparty and collateral netting, liabilities (25) (183)    
Derivative Liability 3,480 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 6,345 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 0 (2,574)    
Counterparty and collateral netting, liabilities (590) (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 (42,825) (42,550)    
Derivative Asset 111 1,264    
Counterparty and collateral netting, liabilities (64,606) (69,988)    
Derivative Liability 1,527 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 0    
Derivative Liability 5 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 (16) (1)    
Derivative Asset 0 31    
Counterparty and collateral netting, liabilities (16) (1)    
Derivative Liability 5 0    
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Total 8,338 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 12,314 7,477    
Total 55,266 51,323    
Interest rate swaps 5,491 73,513    
Total 71,645 144,856    
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Energy commodity derivatives        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Gross Asset 42,936 43,814    
Liability 66,133 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 16 32    
Liability 21 1    
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Total 25 183    
Total 9,855 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 25 183    
Liability 3,505 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 6,345 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 5 19    
Fixed Income Funds [Member] | Fair Value, Measurements, Recurring [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Deferred compensation assets: 1,850 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: 1,850 1,638    
Equity Funds [Member] | Fair Value, Measurements, Recurring [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Deferred compensation assets: 6,488 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: $ 6,488 $ 6,631