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Property, Plant and Equipment (Narrative) (Details) $ in Millions | 12 Months Ended
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Dec. 31, 2015 USD ($) MW | Dec. 31, 2014 USD ($) | Dec. 31, 2013 USD ($)
Property, Plant and Equipment [Line Items] |  |  |  |
Construction work in progress |  | $ 897.5 | $ 479.1 |
Allowance for funds used during construction |  | 36.9 | 34.8 | $ 30.8
Non-regulated Generation, net | [1] | 229.3 | 240.1 |
Corporate Services and other, net | [2] | 319.6 | 269.4 |
IPL [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Construction work in progress |  | 578.2 | 325.0 |
Allowance for funds used during construction |  | $ 28.2 | 25.9 | 21.0
Wholesale power supply agreement termination notice term |  | 5 years |  |
Wholesale power supply agreement term |  | 10 years |  |
WPL [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Construction work in progress |  | $ 319.3 | 154.1 |
Allowance for funds used during construction |  | $ 8.7 | 8.9 | 9.8
Franklin County Wind Farm [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Property, plant and equipment, useful life |  | 30 years |  |
Sheboygan Falls Energy Facility [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Property, plant and equipment, useful life |  | 35 years |  |
Non-regulated Generation, net |  | $ 99.0 |  |
Corporate Services [Member] | Minimum [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Property, plant and equipment, useful life |  | 5 years |  |
Corporate Services [Member] | Maximum [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Property, plant and equipment, useful life |  | 30 years |  |
IPL (FERC formula) [Member] | IPL [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Authorized return on common equity |  | 10.97% |  |
Edgewater Unit 5 [Member] | Alliant Energy and WPL [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Construction work in progress |  | $ 190.0 | 90.0 |
Allowance for funds used during construction |  | $ 8.0 | 3.0 |
Marshalltown Generating Station [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Fossil-fueled EGU capacity (in megawatts) | MW |  | 650 |  |
Marshalltown Generating Station [Member] | IPL [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Allowance for funds used during construction |  | $ 20.7 | 3.7 | $ 0.0
Marshalltown Generating Station [Member] | Alliant Energy and IPL [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Construction work in progress |  | 453.0 | 188.0 |
Allowance for funds used during construction |  | 24.0 | $ 4.0 |
Electric - distribution [Member] | IPL [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Proceeds from asset sales |  | 129.0 |  |
Electric - distribution [Member] | Alliant Energy and IPL [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Loss on sale of Minnesota distribution assets |  | 11.0 |  |
Gas - distribution [Member] | IPL [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Proceeds from asset sales |  | 11.0 |  |
Receipt of promissory note |  | 2.0 |  |
Gas - distribution [Member] | Alliant Energy and IPL [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Loss on sale of Minnesota distribution assets |  | $ 3.0 |  |
Customer Billing And Information System [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Property, plant and equipment, useful life |  | 12 years |  |
Corporate Services and other, net |  | $ 102.0 |  |
Capitalized interest |  | $ 3.0 |  |
[1] | Less accumulated depreciation of $59.0 million and $49.5 million for Alliant Energy as of December 31, 2015 and 2014, respectively.
[2] | Less accumulated depreciation of $252.9 million and $229.1 million for Alliant Energy as of December 31, 2015 and 2014, respectively.

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