Property, Plant and Equipment Joint Ownership (Details)	12 Months Ended
Dec. 31, 2024 USD ($) plants Rate	Dec. 31, 2023 USD ($)
Jointly Owned Utility Plant Interests [Line Items]
Number of joint ownership interests in electric generating plants \| plants	4
Asset Acquisition, Price of Acquisition, Expected	$ 0
Colstrip Ownership in Unit 3 and 4, effective December 31, 2025 Acquired from Avista Corporation
Jointly Owned Utility Plant Interests [Line Items]
Asset Acquisition, Effective Date of Acquisition	Dec. 31, 2025
Colstrip Ownership in Unit 3 and 4, effective December 31, 2025
Jointly Owned Utility Plant Interests [Line Items]
Ownership percentages \| Rate	55.00%
Big Stone Generating Facility [Member]
Jointly Owned Utility Plant Interests [Line Items]
Ownership percentages	23.40%	23.40%
Plant in service	$ 157,572,000	$ 156,696,000
Accumulated depreciation	$ 49,573,000	$ 44,525,000
Neal 4 Generating Facility [Member]
Jointly Owned Utility Plant Interests [Line Items]
Ownership percentages	8.70%	8.70%
Plant in service	$ 65,426,000	$ 64,132,000
Accumulated depreciation	$ 39,025,000	$ 37,178,000
Coyote Generating Facility [Member]
Jointly Owned Utility Plant Interests [Line Items]
Ownership percentages	10.00%	10.00%
Plant in service	$ 52,430,000	$ 52,630,000
Accumulated depreciation	$ 39,887,000	$ 39,393,000
Colstrip Unit 4 [Member]
Jointly Owned Utility Plant Interests [Line Items]
Ownership percentages	30.00%	30.00%
Plant in service	$ 330,888,000	$ 323,793,000
Accumulated depreciation	$ 137,153,000	$ 127,381,000
Colstrip Ownership in Unit 3 and 4, effective December 31, 2025 Acquired from Avista Corporation
Jointly Owned Utility Plant Interests [Line Items]
Ownership percentages	15.00%
Colstrip Ownership in Unit 3 and 4, effective December 31, 2025 Acquired from Puget
Jointly Owned Utility Plant Interests [Line Items]
Ownership percentages	25.00%

Number Of Joint Ownership Interest Electric Generating Plants

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- Definition

Date when acquirer obtains control of acquired asset, in YYYY-MM-DD format.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 805
-SubTopic 50
-Name Accounting Standards Codification
-Section 15
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480123/805-50-15-3

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 805
-SubTopic 50
-Name Accounting Standards Codification
-Section 25
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480060/805-50-25-1

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Purchase price of expected asset acquisition prior to consideration being transferred. Excludes business acquisition.

+ References

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- Definition

The period end amount owned by the reporting utility of a jointly owned utility plant in service.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 980
-SubTopic 360
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 10.C)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479070/980-360-S99-1

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

The period end amount of accumulated depreciation in a jointly owned electricity generation plant.

+ References

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- Definition

The proportionate share of interest (percent) in a jointly owned utility plant.

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