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Jointly-Owned Facilities and Projects (Details) $ in Millions	Dec. 31, 2018 USD ($) MW	Dec. 31, 2017 USD ($)
Jointly-Owned Facilities and Projects [Line Items]
Plant in Service	$ 751.1	$ 769.2
Accumulated Depreciation	240.9	231.2
Construction Work in Progress	6.4	8.2
Transmission [Member] | CapX2020 Projects [Member]
Jointly-Owned Facilities and Projects [Line Items]
Plant in Service	101.0	101.0
Accumulated Depreciation	11.0	8.4
Construction Work in Progress	$ 0.0	$ 0.0
Transmission [Member] | CapX2020 Projects [Member] | Minimum [Member]
Jointly-Owned Facilities and Projects [Line Items]
Minnesota Power Ownership %	9.30%	9.30%
Transmission [Member] | CapX2020 Projects [Member] | Maximum [Member]
Jointly-Owned Facilities and Projects [Line Items]
Minnesota Power Ownership %	14.70%	14.70%
Boswell Unit 4 [Member] | Jointly-Owned Electricity Generation Plant [Member]
Jointly-Owned Facilities and Projects [Line Items]
Minnesota Power Ownership %	80.00%	80.00%
Generating Capacity Jointly Owned (MW) | MW	585
Plant in Service	$ 650.1	$ 668.2
Accumulated Depreciation	229.9	222.8
Construction Work in Progress	$ 6.4	$ 8.2
Boswell Unit 4 [Member] | Jointly-Owned Electricity Generation Plant [Member] | Minnesota Power [Member]
Jointly-Owned Facilities and Projects [Line Items]
Minnesota Power Ownership %	80.00%
Boswell Unit 4 [Member] | Jointly-Owned Electricity Generation Plant [Member] | WPPI Energy [Member]
Jointly-Owned Facilities and Projects [Line Items]
WPPI Energy Ownership %	20.00%

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Generating Capacity Jointly Owned (MW)

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Jointly-Owned Electric Facility Percentage Not Owned

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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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The period end amount of construction work in progress at a jointly owned electricity generation plant.

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

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

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