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Property, Plant and Equipment (Equity and Debt AFUDC) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Property, Plant and Equipment [Line Items] |  |  |
Allowance for funds used during construction | $ 36.9 | $ 34.8 | $ 30.8
Equity [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Allowance for funds used during construction | 24.4 | 23.1 | 20.3
Debt [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Allowance for funds used during construction | 12.5 | 11.7 | 10.5
IPL [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Allowance for funds used during construction | 28.2 | 25.9 | 21.0
IPL [Member] | Equity [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Allowance for funds used during construction | 18.6 | 17.1 | 13.8
IPL [Member] | Debt [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Allowance for funds used during construction | 9.6 | 8.8 | 7.2
WPL [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Allowance for funds used during construction | 8.7 | 8.9 | 9.8
WPL [Member] | Equity [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Allowance for funds used during construction | 5.8 | 6.0 | 6.5
WPL [Member] | Debt [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Allowance for funds used during construction | $ 2.9 | $ 2.9 | $ 3.3

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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=31042434&loc=d3e3602-108585 + Details
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