Debt (Narrative) (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Debt Instrument [Line Items]
Amount outstanding	$ 70.4		$ 102.8
Proceeds from issuance of long-term debt	75.0	0.4
Alliant Energy and IPL [Member]
Debt Instrument [Line Items]
Remaining maturity (in days)	2 days
Weighted average interest rate	0.40%
Alliant Energy and IPL [Member] | Long-Term Debt [Member]
Debt Instrument [Line Items]
Amount outstanding	50.0
Corporate Services [Domain] | 3.45% Senior Notes Due 2022 [Member]
Debt Instrument [Line Items]
Proceeds from issuance of long-term debt	$ 75.0
Debt instrument, interest rate, stated percentage	3.45%