Debt (Narrative) (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended			9 Months Ended			1 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Sep. 30, 2013 Alliant Energy and IPL [Member] Long-term Debt [Member]	Sep. 30, 2013 IPL [Member]	Dec. 31, 2012 IPL [Member]	Oct. 31, 2013 4.7% senior debentures due 2043 Subsequent Event [Member] IPL [Member] Senior Debentures [Member]
Debt Instrument [Line Items]
Commercial paper	$ 237.3		$ 217.5	$ 65.0	$ 0	$ 26.3
Remaining maturity (in days)				2 days
Weighted average interest rate				0.25%			4.70%
Proceeds from issuance of long-term debt	$ 0	$ 75.0					$ 250.0