Property, Plant and Equipment (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended			3 Months Ended		9 Months Ended			9 Months Ended						12 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013	Sep. 30, 2014 IPL [Member]	Sep. 30, 2013 IPL [Member]	Sep. 30, 2014 IPL [Member]	Sep. 30, 2013 IPL [Member]	Dec. 31, 2013 IPL [Member]	Sep. 30, 2014 George Neal Unit 3 [Member] Alliant Energy and IPL [Member]	Sep. 30, 2014 Ottumwa Unit 1 [Member] Alliant Energy and IPL [Member]	Sep. 30, 2014 Columbia Units 1 and 2 [Member] Alliant Energy and WPL [Member]	Sep. 30, 2014 Edgewater Unit 5 [Member] Alliant Energy and WPL [Member]	Sep. 30, 2014 Marshalltown [Member] MW	Sep. 30, 2014 Marshalltown [Member] Alliant Energy and IPL [Member]	Dec. 31, 2014 Gas distribution assets [Member] Scenario, Forecast [Member] IPL [Member]
Property, Plant and Equipment [Line Items]
Electric plant	$ 10,016.3		$ 10,016.3		$ 9,415.7	$ 5,252.6		$ 5,252.6		$ 5,034.9	$ 59.0	$ 151.0	$ 272.0	$ 67.0		$ 120.0
Allowance for funds used during construction	8.3	8.5	25.8	21.1		6.6	5.8	18.6	14.3		4.0	18.0	15.0	2.0		2.0
Fossil-fueled EGU capacity (in megawatts)															650
Expected proceeds from asset sales																	$ 10