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Debt (Narrative) (Details) (USD $) In Millions, unless otherwise specified			3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 Alliant Energy and IPL [Member] Long-term Debt [Member]
Debt Instrument [Line Items]
Amount outstanding	$ 243.4	$ 217.5	$ 35.0
Remaining maturity (in days)			3 days
Weighted average interest rate			0.30%

Carrying value as of the balance sheet date of short-term borrowings using unsecured obligations issued by banks, corporations and other borrowers to investors. The maturities of these money market securities generally do not exceed 270 days.

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