Jointly-Owned Facilities and Assets (Details) $ in Millions	Dec. 31, 2020 USD ($) MW	Dec. 31, 2019 USD ($)
Jointly-Owned Facilities and Assets [Line Items]
Plant in Service	$ 764.8	$ 763.7
Accumulated Depreciation	304.8	272.4
Construction Work in Progress	15.0	5.7
Transmission [Member] \| CapX2020 Projects [Member]
Jointly-Owned Facilities and Assets [Line Items]
Plant in Service	101.0	101.0
Accumulated Depreciation	16.0	13.5
Construction Work in Progress	$ 0.0	$ 0.0
Transmission [Member] \| CapX2020 Projects [Member] \| Minimum [Member]
Jointly-Owned Facilities and Assets [Line Items]
Minnesota Power Ownership %	9.30%	9.30%
Transmission [Member] \| CapX2020 Projects [Member] \| Maximum [Member]
Jointly-Owned Facilities and Assets [Line Items]
Minnesota Power Ownership %	14.70%	14.70%
Boswell Unit 4 [Member] \| Jointly-Owned Electricity Generation Plant [Member]
Jointly-Owned Facilities and Assets [Line Items]
Minnesota Power Ownership %	80.00%	80.00%
Generating Capacity Jointly Owned (MW) \| MW	585
Plant in Service	$ 663.8	$ 662.7
Accumulated Depreciation	288.8	258.9
Construction Work in Progress	$ 15.0	$ 5.7
Boswell Unit 4 [Member] \| Jointly-Owned Electricity Generation Plant [Member] \| Minnesota Power [Member]
Jointly-Owned Facilities and Assets [Line Items]
Minnesota Power Ownership %	80.00%
Boswell Unit 4 [Member] \| Jointly-Owned Electricity Generation Plant [Member] \| WPPI Energy [Member]
Jointly-Owned Facilities and Assets [Line Items]
WPPI Energy Ownership %	20.00%

Generating Capacity Jointly Owned (MW)

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- Definition

Jointly-Owned Electric Facility Percentage Not Owned

+ References

No definition available.

+ Details

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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

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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 construction work in progress at a jointly owned electricity generation plant.

+ References

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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

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