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.