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Property, Plant and Equipment (Equity and Debt AFUDC) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Property, Plant and Equipment [Line Items] |  |  |
Allowance for funds used during construction | $ 92.7 | $ 75.6 | $ 49.7
Equity [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Allowance for funds used during construction | 65.5 | 51.4 | 33.6
Debt [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Allowance for funds used during construction | 27.2 | 24.2 | 16.1
IPL [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Allowance for funds used during construction | 49.4 | 42.2 | 31.4
IPL [Member] | Equity [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Allowance for funds used during construction | 35.2 | 28.6 | 21.1
IPL [Member] | Debt [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Allowance for funds used during construction | 14.2 | 13.6 | 10.3
WPL [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Allowance for funds used during construction | 43.3 | 33.4 | 18.3
WPL [Member] | Equity [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Allowance for funds used during construction | 30.3 | 22.8 | 12.5
WPL [Member] | Debt [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Allowance for funds used during construction | $ 13.0 | $ 10.6 | $ 5.8

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- Definition Total increase in earnings in the period representing the cost of equity (based on assumed rate of return) and/or borrowed funds (based on interest rate) used to finance construction of regulated assets, which is expected to be recovered through rate adjustments. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3602-108585 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44278-110382 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 360 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6500269&loc=d3e45485-110386 + Details
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