Redeemable Preferred Stock (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended				12 Months Ended	1 Months Ended		3 Months Ended				12 Months Ended	1 Months Ended			3 Months Ended	1 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013 IPL [Member]	Mar. 31, 2012 IPL [Member]	Dec. 31, 2012 IPL [Member]	Mar. 31, 2013 IPL [Member] Redeemable Preferred Stock [Member] 5.1% [Member]	Mar. 31, 2013 IPL [Member] Redeemable Preferred Stock [Member] 8.375% [Member]	Mar. 31, 2013 Alliant Energy and IPL [Member] Redeemable Preferred Stock [Member] 5.1% [Member]	Mar. 31, 2013 Alliant Energy and IPL [Member] Redeemable Preferred Stock [Member] 8.375% [Member]	Mar. 31, 2013 WPL [Member]	Mar. 31, 2012 WPL [Member]	Dec. 31, 2012 WPL [Member]	Mar. 31, 2013 WPL [Member] Redeemable Preferred Stock [Member] 4.40% [Member]	Mar. 31, 2013 WPL [Member] Redeemable Preferred Stock [Member] 6.50% [Member]	Mar. 31, 2013 WPL [Member] Redeemable Preferred Stock [Member] 4.40% through 6.50% [Member]	Mar. 31, 2013 Alliant Energy and WPL [Member] Redeemable Preferred Stock [Member] 4.40% through 6.50% [Member]	Mar. 31, 2013 Long-term Debt [Member] IPL [Member] Redeemable Preferred Stock [Member] 5.1% [Member]
Temporary Equity [Line Items]
Preferred stock issued during period (in shares)						8,000,000
Preferred stock series			5.10%		8.375%	5.10%	8.375%					4.50%	4.40%	6.50%
Preferred stock issued during period, value						$ 200
Reduction in commercial paper	10.9	(20.8)	(41.3)	17.9						73.4	(2.4)						40.0
Preferred stock issuance costs								5
Preferred stock redemption terms						On or after March 15, 2018
Preferred stock redemption price (in dollars per share)						$ 25
Preferred stock redeemed during period (in shares)							6,000,000								1,049,225
Stock redeemed during period, value							150								61
Preferred stock issuance charge	$ 10.2	$ 4.0	$ 8.6	$ 3.2					$ 5.0	$ 1.6	$ 0.8					$ 1.0