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Commitments and Contingencies - Legal Contingencies (Details)
 1 Months Ended 3 Months Ended 9 Months Ended 12 Months Ended 27 Months Ended
Jan. 01, 2024
Oct. 01, 2021
USD ($)
Aug. 18, 2017
USD ($)
Sep. 30, 2017
USD ($)
Sep. 30, 2021
USD ($)
Sep. 30, 2020
USD ($)
Sep. 30, 2021
USD ($)
Sep. 30, 2020
USD ($)
Dec. 31, 2023
USD ($)
MW
Dec. 31, 2013
USD ($)
Dec. 31, 2023
USD ($)
MW
Sep. 30, 2022
MW
Jul. 31, 2021
MW
Ravenswood Retiree Medical Benefits Changes                          
Loss Contingencies [Line Items]                          
Litigation settlement amount     $ 23,000,000                    
Litigation settlement period                 10 years        
Ravenswood retiree legal settlement       $ 5,000,000                  
Gain (loss) related to litigation settlement       5,500,000                  
Loss contingency accrual       $ 12,500,000                  
Litigation settlement, annual installment                 $ 2,000,000        
Litigation settlement installment period                 9 years        
Loss contingency, accrual, current                 $ 2,000,000   $ 2,000,000    
Other liabilities                 $ 3,300,000   3,300,000    
Pension Benefit Guarantee Corporation                          
Loss Contingencies [Line Items]                          
Required pension contributions above minimum                   $ 17,400,000      
Pension contributions         $ 0 $ 0 $ 0 $ 0     $ 6,900,000    
Pension contributions, amended term, annual contribution   $ 2,400,000                      
Pension contributions, amended term, total contribution   $ 9,600,000                      
Pension contributions, term   4 years                      
Grundartangi - HS, Landsvirkjun                          
Loss Contingencies [Line Items]                          
Underlying, derivative power, available | MW                 545   545    
Grundartangi - Landsvirkjun                          
Loss Contingencies [Line Items]                          
Underlying, derivative power, available | MW                       25  
Underlying, derivative power, available, extension | MW                       42 161
Underlying, derivative power, available, requested | MW                         182
Underlying, derivative power, rate | MW                       119  
Subsequent Event | Mt. Holly                          
Loss Contingencies [Line Items]                          
Short-term power agreement, supply percentage 100.00%