Commitments and Contingencies - Additional Information (Details)	1 Months Ended			6 Months Ended
	Apr. 30, 2022 USD ($) a	Jul. 31, 2021 USD ($)	Aug. 31, 2019 USD ($)	Jun. 30, 2022 Lawsuit Plaintiff	May 31, 2021 a Building	Sep. 02, 2020 Building
Avista Corp [Member]
Loss Contingencies [Line Items]
Percentage Of Employees, Collective Bargaining Agreement				40.00%
Majority Of Bargaining Unit Employees, Percentage				90.00%
Number of collective bargaining agreement term				4 years
Natural and Cultural Damage Claim [Member]
Loss Contingencies [Line Items]
Litigation settlement damages claim amount		$ 2,000,000
Boyds Fire [Member] | Damage from Fire [Member] | Maximum [Member]
Loss Contingencies [Line Items]
Loss contingency, damages sought, value			$ 4,400,000
Labor Day 2020 Windstorm [Member] | Damage from Fire [Member]
Loss Contingencies [Line Items]
Number of residential, commercial and other structures impacted | Building						230
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
Babb Road Fire [Member] | Damage from Fire [Member]
Loss Contingencies [Line Items]
Number of residential, commercial and other structures impacted | Building					220
Road fire covered area | a					15,000
Number of lawsuits filed seeking unspecified damages | Lawsuit				7
Number of subrogation actions filed | Lawsuit				4
Number of actions on behalf of individual plaintiffs | Plaintiff				2
Number of class action lawsuit | Lawsuit				1
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				These business disagreements, in turn, have led to disagreements as to the interpretation of the O&O Agreement, including, but not limited to, whether a 55 percent vote of the Owner’s Committee is sufficient or whether unanimous consent of the owners is required to either remove a Colstrip unit from service or make a determination that the project can no longer be operated consistent with prudent utility practice or the requirements of governmental agencies having jurisdiction. 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. Both the arbitration and these legal proceedings remain pending.
Colstrip [Member] | Avista Corp [Member]
Loss Contingencies [Line Items]
Owners percentage interest				55.00%