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Property, Plant and Equipment (Narrative) (Details) $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2017 USD ($) MW | Mar. 31, 2016 USD ($) | Apr. 30, 2017 USD ($)
Property, Plant and Equipment [Line Items] |  |  |
Allowance for funds used during construction | $ 17.0 | $ 13.2 |
IPL [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Allowance for funds used during construction | 14.3 | 10.3 |
WPL [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Allowance for funds used during construction | $ 2.7 | $ 2.9 |
Marshalltown [Member] | IPL [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Fossil-fueled EGU capacity (in megawatts) | MW | 660 |  |
Marshalltown [Member] | Alliant Energy and IPL [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Capitalized expenditures | $ 635.0 |  |
Allowance for funds used during construction | $ 79.0 |  |
Riverside Energy Center Expansion [Member] | WPL [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Fossil-fueled EGU capacity (in megawatts) | MW | 730 |  |
Riverside Energy Center Expansion [Member] | Alliant Energy and WPL [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Allowance for funds used during construction | $ 3.0 |  |
Construction work in progress | $ 129.0 |  |
Subsequent Event [Member] | Franklin County Wind Farm [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Value of Franklin County wind farm |  |  | $ 33.0

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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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