Commitments and Contingencies - Additional Information (Details)	1 Months Ended				6 Months Ended
	Apr. 30, 2024 USD ($)	Apr. 30, 2022 USD ($) a	Jul. 31, 2021 USD ($)	Aug. 31, 2019 USD ($)	Jun. 30, 2024 USD ($) Lawsuit Plaintiff	Jul. 31, 2024	Aug. 31, 2023 a	Jan. 16, 2023	Sep. 30, 2020 a Structures
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
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					29
Boyds Fire [Member] | Damage from Fire [Member] | Maximum [Member]
Loss Contingencies [Line Items]
Loss contingency, damages sought, value				$ 4,400,000
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					$ 5,100,000
Insurance Settlements Receivable					$ 2,400,000
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 ultimate 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%
Colstrip [Member] | PSE [Member] | Unit 4 [Member] | Subsequent Event [Member]
Loss Contingencies [Line Items]
Owners percentage interest						25.00%
Colstrip [Member] | NorthWestern [Member] | Unit 4 [Member]
Loss Contingencies [Line Items]
Owners percentage interest					30.00%			15.00%