Property, Plant and Equipment Joint Ownership (Details)	12 Months Ended
Dec. 31, 2022 USD ($) plants	Dec. 31, 2021 USD ($)
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
Big Stone Generating Facility [Member]
Jointly Owned Utility Plant Interests [Line Items]
Ownership percentages	23.40%	23.40%
Plant in service	$ 155,567,000	$ 154,375,000
Accumulated depreciation	$ 42,884,000	$ 42,102,000
Neal 4 Generating Facility [Member]
Jointly Owned Utility Plant Interests [Line Items]
Ownership percentages	8.70%	8.70%
Plant in service	$ 63,032,000	$ 62,865,000
Accumulated depreciation	$ 35,847,000	$ 34,629,000
Coyote Generating Facility [Member]
Jointly Owned Utility Plant Interests [Line Items]
Ownership percentages	10.00%	10.00%
Plant in service	$ 51,796,000	$ 51,652,000
Accumulated depreciation	$ 38,955,000	$ 38,453,000
Colstrip Unit 4 [Member]
Jointly Owned Utility Plant Interests [Line Items]
Ownership percentages	30.00%	30.00%
Plant in service	$ 326,584,000	$ 324,433,000
Accumulated depreciation	$ 121,830,000	$ 113,805,000
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%

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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 50
-Section 15
-Paragraph 3
-URI https://asc.fasb.org/extlink&oid=6911878&loc=d3e8732-128492

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 50
-Section 25
-Paragraph 1
-URI https://asc.fasb.org/extlink&oid=127000608&loc=d3e9135-128495

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

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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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 360
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 10.C)
-URI https://asc.fasb.org/extlink&oid=122136288&loc=d3e659983-123033

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+ References

No definition available.

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

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- Definition

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

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