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Property, Plant and Equipment (Narrative) (Details) $ / shares in Units, $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2016 USD ($) $ / shares MW | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($)
Property, Plant and Equipment [Line Items] |  |  |  |
Construction work in progress |  | $ 1,226.8 | $ 897.5 |
Allowance for funds used during construction |  | 62.5 | 36.9 | $ 34.8
Asset valuation charges, pre-tax |  | 86.4 | 0.0 | 0.0
Non-regulated Generation, net | [1] | 135.0 | 229.3 |
IPL [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Construction work in progress |  | 968.1 | 578.2 |
Allowance for funds used during construction |  | $ 52.0 | 28.2 | 25.9
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 |  | $ 258.7 | 319.3 |
Allowance for funds used during construction |  | $ 10.5 | 8.7 | 8.9
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 |  |
Edgewater Unit 5 [Member] | Alliant Energy and WPL [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Electric plant in service |  | $ 225.0 |  |
Construction work in progress |  |  | 190.0 |
Allowance for funds used during construction |  | 12.0 | 8.0 |
Marshalltown Generating Station [Member] | IPL [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Allowance for funds used during construction |  | $ 43.8 | 20.7 | $ 3.7
Fossil-fueled EGU capacity (in megawatts) | MW |  | 650 |  |
Marshalltown Generating Station [Member] | Alliant Energy and IPL [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Construction work in progress |  | $ 612.0 | 453.0 |
Allowance for funds used during construction |  | 68.0 | 24.0 |
Riverside Energy Center Expansion [Member] | Alliant Energy and WPL [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Construction work in progress |  | 81.0 | 2.0 |
Allowance for funds used during construction |  | $ 2.0 | 0.0 |
Riverside Energy Center Expansion [Member] | WPL [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Fossil-fueled EGU capacity (in megawatts) | MW |  | 730 |  |
Expansion of Wind Generation [Member] | Alliant Energy and IPL [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Construction work in progress |  | $ 102.0 |  |
Allowance for funds used during construction |  | $ 1.0 |  |
IPL (FERC formula) [Member] | IPL [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Authorized return on common equity |  | 10.97% |  |
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 |
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 |
Franklin County Wind Farm [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Property, plant and equipment, useful life |  | 30 years |  |
Value of Franklin County wind farm |  | $ 33.0 |  |
Asset valuation charges, pre-tax |  | 86.0 |  |
Asset valuation charges, after-tax |  | $ 51.0 |  |
Asset valuation charges, after-tax (in dollars per share) | $ / shares |  | $ 0.23 |  |
Sheboygan Falls Energy Facility [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Property, plant and equipment, useful life |  | 35 years |  |
Non-regulated Generation, net |  | $ 95.0 |  |
Customer Billing And Information System [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Property, plant and equipment, useful life |  | 12 years |  |
Software [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Property, plant and equipment, useful life |  | 5 years |  |
[1] | Less accumulated depreciation of $46.5 million and $59.0 million for Alliant Energy as of December 31, 2016 and 2015, respectively.

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