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Property, Plant and Equipment (Narrative) (Details) $ / shares in Units, $ in Millions | 1 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Apr. 30, 2018 USD ($) MW | Dec. 31, 2018 USD ($) MW | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) $ / shares | Apr. 30, 2017 USD ($)
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Asset valuation charges, pre-tax |  |  | $ 0.0 | $ 0.0 | $ 86.4 |
Non-utility Generation, net | [1] |  | $ 86.9 | $ 90.9 |  |
IPL [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Net assets acquired |  |  |  |  |  | $ 32.0
WPL [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Net assets acquired |  | $ 74.0 |  |  |  |
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 |  |  |
Expansion of Wind Generation [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Electric capacity of wind farm (in megawatts) | MW |  |  | 129 |  |  |
Expansion of Wind Generation [Member] | WPL [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Electric capacity of wind farm (in megawatts) | MW |  | 55 |  |  |  |
Net assets acquired |  | $ 74.0 |  |  |  |
Franklin County Wind Farm [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
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-utility Generation, net |  |  | $ 87.0 |  |  |
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 $54.5 million and $50.5 million for Alliant Energy as of December 31, 2018 and 2017, respectively.

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