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Fair Value (Narrative) (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2024
Jun. 30, 2023
Jun. 30, 2024
Jun. 30, 2023
Mar. 31, 2024
Dec. 31, 2023
Mar. 31, 2023
Dec. 31, 2022
Fair Value, Option, Quantitative Disclosures [Line Items]                
Investments, Fair Value Disclosure $ 3,725 $ 3,552 $ 3,725 $ 3,552        
Asset Retirement Obligation, Revision of Estimate     80,221 0        
Asset retirement obligations 342,540   342,540     $ 249,930    
Unamortized deferred financing costs 35,500   35,500     24,800    
Unamortized debt discount 9,600   9,600     6,800    
Financial Transmission Rights Fair Value Disclosure 3,968 1,388 3,968 1,388        
Assets, Fair Value Disclosure 7,693 19,234 7,693 19,234        
Forward Power Contracts Fair Value Disclosure 0 14,294 0 14,294        
Derivative Financial Instruments, Liabilities                
Fair Value, Option, Quantitative Disclosures [Line Items]                
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value 3,968 3,294 3,968 3,294 $ 393 $ 1,388 $ 2,559 $ 7,545
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances 3,811 3,624 3,811 3,624        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements (236) (2,889) (1,231) (7,875)        
Money Market Funds [Member]                
Fair Value, Option, Quantitative Disclosures [Line Items]                
Investments, Fair Value Disclosure 87 127 87 127        
Mutual Fund [Member]                
Fair Value, Option, Quantitative Disclosures [Line Items]                
Investments, Fair Value Disclosure 3,638 3,425 3,638 3,425        
Fair Value, Inputs, Level 3 [Member]                
Fair Value, Option, Quantitative Disclosures [Line Items]                
Investments, Fair Value Disclosure 0 0 0 0        
Financial Transmission Rights Fair Value Disclosure 3,968 1,388 3,968 1,388        
Assets, Fair Value Disclosure 3,968 1,388 3,968 1,388        
Forward Power Contracts Fair Value Disclosure 0 0 0 0        
Fair Value, Inputs, Level 3 [Member] | Money Market Funds [Member]                
Fair Value, Option, Quantitative Disclosures [Line Items]                
Investments, Fair Value Disclosure 0 0 0 0        
Fair Value, Inputs, Level 3 [Member] | Mutual Fund [Member]                
Fair Value, Option, Quantitative Disclosures [Line Items]                
Investments, Fair Value Disclosure 0 0 0 0        
Fair Value, Inputs, Level 2 [Member]                
Fair Value, Option, Quantitative Disclosures [Line Items]                
Investments, Fair Value Disclosure 0 0 0 0        
Financial Transmission Rights Fair Value Disclosure 0 0 0 0        
Assets, Fair Value Disclosure 0 14,294 0 14,294        
Forward Power Contracts Fair Value Disclosure 0 14,294 0 14,294        
Fair Value, Inputs, Level 2 [Member] | Money Market Funds [Member]                
Fair Value, Option, Quantitative Disclosures [Line Items]                
Investments, Fair Value Disclosure 0 0 0 0        
Fair Value, Inputs, Level 2 [Member] | Mutual Fund [Member]                
Fair Value, Option, Quantitative Disclosures [Line Items]                
Investments, Fair Value Disclosure 0 0 0 0        
Fair Value, Inputs, Level 1 [Member]                
Fair Value, Option, Quantitative Disclosures [Line Items]                
Investments, Fair Value Disclosure 3,725 3,552 3,725 3,552        
Financial Transmission Rights Fair Value Disclosure 0 0 0 0        
Assets, Fair Value Disclosure 3,725 3,552 3,725 3,552        
Forward Power Contracts Fair Value Disclosure 0 0 0 0        
Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member]                
Fair Value, Option, Quantitative Disclosures [Line Items]                
Investments, Fair Value Disclosure 87 127 87 127        
Fair Value, Inputs, Level 1 [Member] | Mutual Fund [Member]                
Fair Value, Option, Quantitative Disclosures [Line Items]                
Investments, Fair Value Disclosure 3,638 $ 3,425 3,638 $ 3,425        
Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]                
Fair Value, Option, Quantitative Disclosures [Line Items]                
Derivative, Notional Amount $ 400,000   $ 400,000