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Property, Plant and Equipment (Narrative) (Details) $ in Millions | 1 Months Ended | 3 Months Ended | 9 Months Ended
--- | --- | --- | ---
Jul. 31, 2015 USD ($) | Apr. 30, 2015 USD ($) | Sep. 30, 2015 USD ($) | Sep. 30, 2014 USD ($) | Sep. 30, 2015 USD ($) MW | Sep. 30, 2014 USD ($)
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Allowance for funds used during construction |  |  | $ 9.7 | $ 8.3 | $ 25.1 | $ 25.8
IPL [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Allowance for funds used during construction |  |  | 7.3 | $ 6.6 | 19.3 | $ 18.6
Edgewater Unit 5 [Member] | Alliant Energy and WPL [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Construction work in progress |  |  | 171.0 |  | 171.0 |
Allowance for funds used during construction |  |  |  |  | $ 6.0 |
Marshalltown [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Fossil-fueled EGU capacity (in megawatts) | MW |  |  |  |  | 650 |
Marshalltown [Member] | Alliant Energy and IPL [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Construction work in progress |  |  | 414.0 |  | $ 414.0 |
Allowance for funds used during construction |  |  |  |  | 17.0 |
Gas distribution assets [Member] | Alliant Energy and IPL [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Loss on sale of Minnesota distribution assets |  |  |  |  | 3.0 |
Gas distribution assets [Member] | IPL [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Proceeds from asset sales |  | $ 11.0 |  |  |  |
Receipt of promissory note |  | $ 2.0 |  |  |  |
Electric distribution assets [Member] | Alliant Energy and IPL [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Loss on sale of Minnesota distribution assets |  |  |  |  | 9.0 |
Electric distribution assets [Member] | IPL [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Proceeds from asset sales | $ 127.0 |  |  |  |  |
Customer Billing And Information System [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Capitalized expenditures |  |  | 87.0 |  | 87.0 |
Capitalized interest |  |  | $ 2.0 |  | $ 2.0 |

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