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Fair Value (Fair Value Of Assets And Liabilities Measured On Recurring Basis) (Details) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Mar. 31, 2017
Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Gross Asset $ 74,809 $ 51,506    
Liability 145,975 161,467    
Cash and cash equivalents 26,273 16,172 $ 26,179 $ 8,507
Fixed Income Securities [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Cash and cash equivalents 200 200    
Fair Value, Measurements, Recurring [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Counterparty and collateral netting, assets (60,748) (45,308)    
Interest rate swaps 13,084 4,903    
Total 22,534 14,467    
Counterparty and collateral netting, liabilities (104,974) (107,716)    
Interest rate swaps 26,470 35,969    
Total 41,001 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 (47) (183)    
Derivative Asset 0 0    
Counterparty and collateral netting, liabilities (47) (183)    
Derivative Liability 2,805 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 10,163 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 (1,631) (2,574)    
Counterparty and collateral netting, liabilities (29,261) (37,544)    
Fair Value, Measurements, Recurring [Member] | Energy commodity derivatives [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Counterparty and collateral netting, assets (59,064) (42,550)    
Derivative Asset 961 1,264    
Counterparty and collateral netting, liabilities (75,660) (69,988)    
Derivative Liability 1,558 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 (6) (1)    
Derivative Asset 16 31    
Counterparty and collateral netting, liabilities (6) (1)    
Derivative Liability 0 0    
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Total 8,473 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 14,715 7,477    
Total 74,762 51,323    
Interest rate swaps 55,731 73,513    
Total 132,955 144,856    
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Energy commodity derivatives [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Gross Asset 60,025 43,814    
Liability 77,218 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 22 32    
Liability 6 1    
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Total 47 183    
Total 13,020 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 47 183    
Liability 2,852 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 10,163 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: 1,663 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: 1,663 1,638    
Equity Funds [Member] | Fair Value, Measurements, Recurring [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Deferred compensation assets: 6,810 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: $ 6,810 $ 6,631