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Property, Plant and Equipment (Equity and Debt AFUDC) (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Property, Plant and Equipment [Line Items] |  |  |
Allowance for funds used during construction | $ 49.7 | $ 62.5 | $ 36.9
Debt [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Allowance for funds used during construction | 16.1 | 20.2 | 12.5
Equity [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Allowance for funds used during construction | 33.6 | 42.3 | 24.4
IPL [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Allowance for funds used during construction | 31.4 | 52.0 | 28.2
IPL [Member] | Debt [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Allowance for funds used during construction | 10.3 | 16.8 | 9.6
IPL [Member] | Equity [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Allowance for funds used during construction | 21.1 | 35.2 | 18.6
WPL [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Allowance for funds used during construction | 18.3 | 10.5 | 8.7
WPL [Member] | Debt [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Allowance for funds used during construction | 5.8 | 3.4 | 2.9
WPL [Member] | Equity [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Allowance for funds used during construction | $ 12.5 | $ 7.1 | $ 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://www.xbrl.org/2003/role/presentationRef -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 2: http://www.xbrl.org/2003/role/presentationRef -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 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3602-108585 + Details
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