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Property, Plant and Equipment (Narrative) (Details) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2016 USD ($) $ / shares	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) $ / shares MW	Sep. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	$ (15.8)	$ (9.7)	$ (44.3)	$ (25.1)
Current value of Franklin County wind farm assets	33.0		33.0
Asset valuation charges for Franklin County wind farm, pre-tax	86.4	0.0	86.4	0.0
Asset valuation charges for Franklin County wind farm, after tax	$ 51.0		$ 51.0
Asset valuation charges for Franklin County wind farm, after tax (in dollars per share) | $ / shares	$ 0.23		$ 0.23
IPL [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction	$ (13.8)	$ (7.3)	$ (36.2)	(19.3)
Edgewater Unit 5 [Member] | Alliant Energy and WPL [Member]
Property, Plant and Equipment [Line Items]
Plant in service	223.0		223.0
Allowance for funds used during construction			(12.0)
Marshalltown [Member] | Alliant Energy and IPL [Member]
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction			(56.0)
Construction work in progress	$ 600.0		$ 600.0
Marshalltown [Member] | IPL [Member]
Property, Plant and Equipment [Line Items]
Fossil-fueled EGU capacity (in megawatts) | MW			650
Gas Distribution Equipment [Member] | Alliant Energy and IPL [Member]
Property, Plant and Equipment [Line Items]
Loss on sale of Minnesota distribution assets				3.0
Gas Distribution Equipment [Member] | IPL [Member]
Property, Plant and Equipment [Line Items]
Proceeds from sale of property, plant, and equipment					$ 11.0
Receipt of promissory note					2.0
Electric Distribution [Member] | Alliant Energy and IPL [Member]
Property, Plant and Equipment [Line Items]
Loss on sale of Minnesota distribution assets				$ 9.0
Electric Distribution [Member] | IPL [Member]
Property, Plant and Equipment [Line Items]
Proceeds from sale of property, plant, and equipment					$ 129.0

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