Property, Plant and Equipment Joint Ownership (Details) $ in Thousands	Dec. 31, 2019 USD ($) plants	Dec. 31, 2018 USD ($)
Jointly Owned Utility Plant Interests [Line Items]
Number of joint ownership interests in electric generating plants \| plants	4
Big Stone Generating Facility [Member]
Jointly Owned Utility Plant Interests [Line Items]
Ownership percentages	23.40%	23.40%
Plant in service	$ 155,662	$ 155,359
Accumulated depreciation	$ 40,988	$ 42,235
Neal 4 Generating Facility [Member]
Jointly Owned Utility Plant Interests [Line Items]
Ownership percentages	8.70%	8.70%
Plant in service	$ 62,565	$ 60,758
Accumulated depreciation	$ 32,853	$ 31,542
Coyote Generating Facility [Member]
Jointly Owned Utility Plant Interests [Line Items]
Ownership percentages	10.00%	10.00%
Plant in service	$ 52,448	$ 50,325
Accumulated depreciation	$ 38,310	$ 37,955
Colstrip Unit 4 [Member]
Jointly Owned Utility Plant Interests [Line Items]
Ownership percentages	30.00%	30.00%
Plant in service	$ 311,399	$ 309,163
Accumulated depreciation	$ 97,563	$ 88,985

Number Of Joint Ownership Interest Electric Generating Plants

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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 http://asc.fasb.org/extlink&oid=115995737&loc=d3e659983-123033

+ Details

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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.

+ Details

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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.

+ References

+ Details

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