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Commitments and Contingencies - Additional Information (Details)
 1 Months Ended 9 Months Ended
Oct. 31, 2024
Apr. 30, 2024
USD ($)
Apr. 30, 2022
USD ($)
a
Jul. 31, 2021
USD ($)
Aug. 31, 2019
USD ($)
Sep. 30, 2024
USD ($)
Plaintiff
Lawsuit
Dec. 31, 2024
USD ($)
Jul. 31, 2024
Aug. 31, 2023
a
Jan. 16, 2023
Sep. 30, 2020
a
Structures
Subsequent Event [Member]                      
Loss Contingencies [Line Items]                      
Revision period for emissions report 45 days                    
Forecast [Member]                      
Loss Contingencies [Line Items]                      
Additions emissions liability             $ 200,000        
Additional costs allocated percentage             10.00%        
Forecast [Member] | Idaho [Member]                      
Loss Contingencies [Line Items]                      
Estimated additional liability             $ 12,000,000        
Forecast [Member] | Washington [Member]                      
Loss Contingencies [Line Items]                      
Estimated additional liability             20,000,000        
Collective Bargaining Agreements Avista Utilities Employees [Member] | IBEW [Member]                      
Loss Contingencies [Line Items]                      
Percentage of employees represented           36.00%          
Collective Bargaining Agreements Avista Utilities Bargaining Unit Employees [Member] | IBEW [Member]                      
Loss Contingencies [Line Items]                      
Percentage of largest group of employees represented           90.00%          
Collective Bargaining Agreements [Member] | IBEW [Member]                      
Loss Contingencies [Line Items]                      
Agreement term           4 years          
Agreement expiration month and year           2025-03          
Maximum [Member] | Forecast [Member]                      
Loss Contingencies [Line Items]                      
Estimated additional liability             $ 32,000,000        
Customers [Member] | Forecast [Member]                      
Loss Contingencies [Line Items]                      
Additional costs allocated percentage             90.00%        
Natural and Cultural Damage Claim [Member]                      
Loss Contingencies [Line Items]                      
Loss contingency, damages sought, value       $ 2,000,000              
Amout of settle all claims and allegations   $ 900,000                  
Unspecified Damages [Member]                      
Loss Contingencies [Line Items]                      
Number Of Plaintiffs | Plaintiff           128          
Economic Damage [Member]                      
Loss Contingencies [Line Items]                      
Number Of Plaintiffs | Plaintiff           40          
Boyds Fire [Member] | Damage from Fire [Member] | Maximum [Member]                      
Loss Contingencies [Line Items]                      
Loss contingency, damages sought, value         $ 4,400,000            
Boyds Fire [Member] | Property Damages [Member] | Avista Corp [Member]                      
Loss Contingencies [Line Items]                      
Loss contingency, damages sought, value         800,000            
Boyds Fire [Member] | Insurance proceeds paid to their insureds [Member] | Avista Corp [Member]                      
Loss Contingencies [Line Items]                      
Loss contingency, damages sought, value         $ 1,800,000            
Road 11 Fire [Member] | Damage from Fire [Member] | Avista Corp [Member]                      
Loss Contingencies [Line Items]                      
Loss contingency, damages sought, value     $ 5,000,000                
Road fire covered area | a     10,000                
Settlement Amount     $ 100,000                
Babb Road Fire [Member] | Damage from Fire [Member]                      
Loss Contingencies [Line Items]                      
Loss contingency, damages sought, value           $ 23,000,000          
Number of residential, commercial and other structures impacted | Structures                     220
Road fire covered area | a                     15,000
Number of subrogation actions filed | Lawsuit           6          
Number of actions on behalf of individual plaintiffs | Plaintiff           5          
Economic Losses Of Plaintiffs           $ 8,200,000          
Insurance Settlements Receivable           $ 3,500,000          
Number of class action lawsuit | Lawsuit           1          
Orofino Fire [Member] | Damage from Fire [Member]                      
Loss Contingencies [Line Items]                      
Road fire covered area | a                 53    
System Unit Resource Protection Act [Member] | Natural and Cultural Damage Claim [Member]                      
Loss Contingencies [Line Items]                      
Increased potential claim       $ 6,000,000              
Colstrip [Member]                      
Loss Contingencies [Line Items]                      
Agreement voting requirement           NorthWestern has initiated arbitration pursuant to the O&O Agreement to resolve these business disagreements, and two actions have been initiated to compel arbitration of those disputes: one by Talen in the Montana Thirteenth Judicial District Court for Yellowstone County, and one by the Western Co-Owners, which is pending in Montana Federal District Court. In light of the ownership transfer agreements, the Colstrip owners agreed to stay both the litigation and the arbitration through March 2024. On April 1, 2024, the agreement to stay lapsed and the parties are now in the process of reengaging in arbitration discussions. An arbitration date has not yet been scheduled. The Company cannot predict the outcome of the arbitration process.          
Colstrip [Member] | Maximum [Member]                      
Loss Contingencies [Line Items]                      
Litigation Settlement           $ 100,000          
Colstrip [Member] | PSE [Member] | Unit 4 [Member]                      
Loss Contingencies [Line Items]                      
Owners percentage interest           25.00%   25.00%      
Colstrip [Member] | NorthWestern [Member] | Unit 4 [Member]                      
Loss Contingencies [Line Items]                      
Owners percentage interest           30.00%       15.00%