Property, Plant and Equipment (Narrative) (Details) $ / shares in Units, $ in Millions		12 Months Ended
		Dec. 31, 2020 MW	Dec. 31, 2018 USD ($) MW	Dec. 31, 2017 USD ($) MW	Dec. 31, 2016 USD ($) $ / shares	Dec. 31, 2015 USD ($)	Apr. 30, 2017 USD ($)
Property, Plant and Equipment [Line Items]
Construction work in progress				$ 962.2	$ 1,226.8
Allowance for funds used during construction				49.7	62.5	$ 36.9
Asset valuation charges, pre-tax				0.0	86.4	0.0
Non-utility Generation, net	[1]			90.9	135.0
IPL [Member]
Property, Plant and Equipment [Line Items]
Construction work in progress				424.4	968.1
Allowance for funds used during construction				$ 31.4	52.0	28.2
Net assets acquired							$ 32.0
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				$ 537.8	258.7
Allowance for funds used during construction				$ 18.3	10.5	8.7
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
Marshalltown Generating Station [Member] | IPL [Member]
Property, Plant and Equipment [Line Items]
Fossil-fueled EGU capacity (in megawatts) | MW				706
Marshalltown Generating Station [Member] | Alliant Energy and IPL [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service				$ 645.0
Construction work in progress					612.0
Allowance for funds used during construction				$ 81.0	68.0
West Riverside Energy Center [Member] | WPL [Member]
Property, Plant and Equipment [Line Items]
Fossil-fueled EGU capacity (in megawatts) | MW				730
West Riverside Energy Center [Member] | Alliant Energy and WPL [Member]
Property, Plant and Equipment [Line Items]
Construction work in progress				$ 339.0	81.0
Allowance for funds used during construction				$ 14.0	2.0
Expansion of Wind Generation [Member] | IPL [Member]
Property, Plant and Equipment [Line Items]
Electric capacity of wind farm (in megawatts) | MW				500
Expansion of Wind Generation [Member] | WPL [Member]
Property, Plant and Equipment [Line Items]
Electric capacity of wind farm (in megawatts) | MW				129
Expansion of Wind Generation [Member] | Alliant Energy and IPL [Member]
Property, Plant and Equipment [Line Items]
Construction work in progress				$ 264.0	102.0
Allowance for funds used during construction				$ 11.0	1.0
IPL (FERC formula) [Member] | IPL [Member]
Property, Plant and Equipment [Line Items]
Authorized return on common equity				10.97%
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
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
Sheboygan Falls Energy Facility [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life				35 years
Non-utility Generation, net				$ 91.0
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
Scenario, Forecast [Member] | Expansion of Wind Generation [Member] | IPL [Member]
Property, Plant and Equipment [Line Items]
Electric capacity of wind farm (in megawatts) | MW		1,000
Scenario, Forecast [Member] | 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

[1] Less accumulated depreciation of $50.5 million and $46.5 million for Alliant Energy as of December 31, 2017 and 2016, respectively. Refer to “Franklin County Wind Farm” below for discussion of the April 2017 transfer of the Franklin County wind farm from AEF to IPL pursuant to a February 2017 FERC order.