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.