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Jointly-Owned Electric Utility Plant (Details) $ in Millions | Dec. 31, 2018 USD ($)
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Jointly Owned Electric Utility Plant [Line Items] |  |
Electric Plant | $ 1,789.4 |
Accumulated Provision for Depreciation | 622.3 |
Construction Work in Progress | 617.0 |
IPL [Member] |  |
Jointly Owned Electric Utility Plant [Line Items] |  |
Electric Plant | 890.3 |
Accumulated Provision for Depreciation | 333.2 |
Construction Work in Progress | 68.7 |
WPL [Member] |  |
Jointly Owned Electric Utility Plant [Line Items] |  |
Electric Plant | 899.1 |
Accumulated Provision for Depreciation | 289.1 |
Construction Work in Progress | $ 548.3 |
Ottumwa Unit 1 [Member] | IPL [Member] |  |
Jointly Owned Electric Utility Plant [Line Items] |  |
Ownership Interest % | 48.00% |
Electric Plant | $ 507.1 |
Accumulated Provision for Depreciation | 161.7 |
Construction Work in Progress | $ 65.3 |
George Neal Unit 4 [Member] | IPL [Member] |  |
Jointly Owned Electric Utility Plant [Line Items] |  |
Ownership Interest % | 25.70% |
Electric Plant | $ 187.3 |
Accumulated Provision for Depreciation | 89.8 |
Construction Work in Progress | $ 1.2 |
George Neal Unit 3 [Member] | IPL [Member] |  |
Jointly Owned Electric Utility Plant [Line Items] |  |
Ownership Interest % | 28.00% |
Electric Plant | $ 156.7 |
Accumulated Provision for Depreciation | 57.8 |
Construction Work in Progress | $ 1.4 |
Louisa Unit 1 [Member] | IPL [Member] |  |
Jointly Owned Electric Utility Plant [Line Items] |  |
Ownership Interest % | 4.00% |
Electric Plant | $ 39.2 |
Accumulated Provision for Depreciation | 23.9 |
Construction Work in Progress | $ 0.8 |
Columbia Units 1-2 [Member] | WPL [Member] |  |
Jointly Owned Electric Utility Plant [Line Items] |  |
Ownership Interest % | 52.50% |
Electric Plant | $ 779.4 |
Accumulated Provision for Depreciation | 247.9 |
Construction Work in Progress | $ 9.8 |
Forward Wind Energy Center [Member] | WPL [Member] |  |
Jointly Owned Electric Utility Plant [Line Items] |  |
Ownership Interest % | 42.60% |
Electric Plant | $ 119.7 |
Accumulated Provision for Depreciation | 41.2 |
Construction Work in Progress | $ 0.1 |
West Riverside [Member] | WPL [Member] |  |
Jointly Owned Electric Utility Plant [Line Items] |  |
Ownership Interest % | 91.80% | [1]
Electric Plant | $ 0.0 | [1]
Accumulated Provision for Depreciation | 0.0 | [1]
Construction Work in Progress | $ 538.4 | [1]
[1] | In January 2018, certain electric cooperatives, which currently have wholesale power supply agreements with WPL, acquired a partial ownership interest in West Riverside.

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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://www.xbrl.org/2003/role/presentationRef -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=6500719&loc=d3e659983-123033 + Details
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- Definition The period end amount of construction work in progress at a jointly owned electricity generation plant. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -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=6500719&loc=d3e659983-123033 + Details
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- Definition The period end amount of accumulated depreciation in a jointly owned electricity generation plant. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -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=6500719&loc=d3e659983-123033 + Details
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- Definition The proportionate share of interest (percent) in a jointly owned utility plant. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -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=6500719&loc=d3e659983-123033 + Details
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