Property, Plant and Equipment (Narrative) (Details) $ in Millions		12 Months Ended
		Dec. 31, 2015 USD ($) MW	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Property, Plant and Equipment [Line Items]
Construction work in progress		$ 897.5	$ 479.1
Allowance for funds used during construction		36.9	34.8	$ 30.8
Non-regulated Generation, net	[1]	229.3	240.1
Corporate Services and other, net	[2]	319.6	269.4
IPL [Member]
Property, Plant and Equipment [Line Items]
Construction work in progress		578.2	325.0
Allowance for funds used during construction		$ 28.2	25.9	21.0
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		$ 319.3	154.1
Allowance for funds used during construction		$ 8.7	8.9	9.8
Franklin County Wind Farm [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life		30 years
Sheboygan Falls Energy Facility [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life		35 years
Non-regulated Generation, net		$ 99.0
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
IPL (FERC formula) [Member] | IPL [Member]
Property, Plant and Equipment [Line Items]
Authorized return on common equity		10.97%
Edgewater Unit 5 [Member] | Alliant Energy and WPL [Member]
Property, Plant and Equipment [Line Items]
Construction work in progress		$ 190.0	90.0
Allowance for funds used during construction		$ 8.0	3.0
Marshalltown Generating Station [Member]
Property, Plant and Equipment [Line Items]
Fossil-fueled EGU capacity (in megawatts) | MW		650
Marshalltown Generating Station [Member] | IPL [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction		$ 20.7	3.7	$ 0.0
Marshalltown Generating Station [Member] | Alliant Energy and IPL [Member]
Property, Plant and Equipment [Line Items]
Construction work in progress		453.0	188.0
Allowance for funds used during construction		24.0	$ 4.0
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
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
Customer Billing And Information System [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life		12 years
Corporate Services and other, net		$ 102.0
Capitalized interest		$ 3.0

[1] Less accumulated depreciation of $59.0 million and $49.5 million for Alliant Energy as of December 31, 2015 and 2014, respectively.

[2] Less accumulated depreciation of $252.9 million and $229.1 million for Alliant Energy as of December 31, 2015 and 2014, respectively.