Property, Plant and Equipment (Narrative) (Details) $ in Millions	1 Months Ended		3 Months Ended		9 Months Ended
	Jul. 31, 2015 USD ($)	Apr. 30, 2015 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) MW	Sep. 30, 2014 USD ($)
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction			$ 9.7	$ 8.3	$ 25.1	$ 25.8
IPL [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction			7.3	$ 6.6	19.3	$ 18.6
Edgewater Unit 5 [Member] | Alliant Energy and WPL [Member]
Property, Plant and Equipment [Line Items]
Construction work in progress			171.0		171.0
Allowance for funds used during construction					$ 6.0
Marshalltown [Member]
Property, Plant and Equipment [Line Items]
Fossil-fueled EGU capacity (in megawatts) | MW					650
Marshalltown [Member] | Alliant Energy and IPL [Member]
Property, Plant and Equipment [Line Items]
Construction work in progress			414.0		$ 414.0
Allowance for funds used during construction					17.0
Gas distribution assets [Member] | Alliant Energy and IPL [Member]
Property, Plant and Equipment [Line Items]
Loss on sale of Minnesota distribution assets					3.0
Gas distribution assets [Member] | IPL [Member]
Property, Plant and Equipment [Line Items]
Proceeds from asset sales		$ 11.0
Receipt of promissory note		$ 2.0
Electric distribution assets [Member] | Alliant Energy and IPL [Member]
Property, Plant and Equipment [Line Items]
Loss on sale of Minnesota distribution assets					9.0
Electric distribution assets [Member] | IPL [Member]
Property, Plant and Equipment [Line Items]
Proceeds from asset sales	$ 127.0
Customer Billing And Information System [Member]
Property, Plant and Equipment [Line Items]
Capitalized expenditures			87.0		87.0
Capitalized interest			$ 2.0		$ 2.0