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Regulatory Environment - MERC Recovery of Natural Gas Costs (Details)
$ in Millions
Dec. 31, 2024
USD ($)
Dec. 31, 2023
USD ($)
Oct. 31, 2022
USD ($)
Aug. 31, 2021
USD ($)
utility
Feb. 28, 2021
USD ($)
Public Utilities, General Disclosures          
Total regulatory assets $ 3,378.7 $ 3,274.7      
Amounts recoverable from customers 39.0 24.9      
Regulatory assets $ 3,339.7 $ 3,249.8      
Minnesota Public Utilities Commission (MPUC) | MERC          
Public Utilities, General Disclosures          
Number of utilities filing a joint proposal | utility       4  
Minnesota Public Utilities Commission (MPUC) | MERC | Energy costs recoverable through rate adjustments          
Public Utilities, General Disclosures          
Total regulatory assets         $ 75.0
Amounts recoverable from customers       $ 10.0  
Recovery period of regulatory asset       12 months  
Minnesota Public Utilities Commission (MPUC) | MERC | MERC extraordinary natural gas costs          
Public Utilities, General Disclosures          
Recovery period of regulatory asset       27 months  
Regulatory assets     $ 62.0 $ 65.0  
Agreed upon reduction to regulatory asset     $ 3.0