Jointly-Owned Electric Utility Plant (Details) $ in Millions	Dec. 31, 2019 USD ($)
Jointly Owned Electric Utility Plant [Line Items]
Electric Plant	$ 1,864.1
Accumulated Provision for Depreciation	670.7
Construction Work in Progress	673.7
IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Electric Plant	966.9
Accumulated Provision for Depreciation	362.4
Construction Work in Progress	20.7
WPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Electric Plant	897.2
Accumulated Provision for Depreciation	308.3
Construction Work in Progress	$ 653.0
Ottumwa Unit 1 [Member] \| IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	48.00%
Electric Plant	$ 571.5
Accumulated Provision for Depreciation	180.8
Construction Work in Progress	$ 18.8
George Neal Unit 4 [Member] \| IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	25.70%
Electric Plant	$ 191.8
Accumulated Provision for Depreciation	93.5
Construction Work in Progress	$ 0.8
George Neal Unit 3 [Member] \| IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	28.00%
Electric Plant	$ 163.7
Accumulated Provision for Depreciation	63.2
Construction Work in Progress	$ 0.9
Louisa Unit 1 [Member] \| IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	4.00%
Electric Plant	$ 39.9
Accumulated Provision for Depreciation	24.9
Construction Work in Progress	$ 0.2
Columbia Units 1-2 [Member] \| WPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	53.30%
Electric Plant	$ 777.0
Accumulated Provision for Depreciation	264.7
Construction Work in Progress	$ 5.5
Forward Wind Energy Center (FWEC) [Member] \| WPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	42.60%
Electric Plant	$ 120.2
Accumulated Provision for Depreciation	43.6
Construction Work in Progress	$ 0.0
West Riverside [Member] \| WPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	91.80%
Electric Plant	$ 0.0
Accumulated Provision for Depreciation	0.0
Construction Work in Progress	$ 647.5

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

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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- Definition

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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