Redeemable Preferred Stock (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended												1 Months Ended	12 Months Ended					12 Months Ended				12 Months Ended				12 Months Ended				12 Months Ended				12 Months Ended				12 Months Ended				12 Months Ended				1 Months Ended	3 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Redeemable Preferred Stock [Member]	Dec. 31, 2011 Redeemable Preferred Stock [Member]	Dec. 31, 2012 IPL [Member]	Dec. 31, 2011 IPL [Member]	Dec. 31, 2010 IPL [Member]	Dec. 31, 2012 IPL [Member] Redeemable Preferred Stock [Member]	Dec. 31, 2011 IPL [Member] Redeemable Preferred Stock [Member]	Dec. 31, 2012 IPL [Member] Redeemable Preferred Stock [Member] 8.375% [Member]	Dec. 31, 2011 IPL [Member] Redeemable Preferred Stock [Member] 8.375% [Member]	Mar. 31, 2013 IPL [Member] Redeemable Preferred Stock [Member] 8.375% [Member] Subsequent Event [Member] Scenario, Forecast [Member]	Dec. 31, 2012 WPL [Member]	Dec. 31, 2011 WPL [Member]	Dec. 31, 2010 WPL [Member]	Dec. 31, 2012 WPL [Member] Redeemable Preferred Stock [Member]	Dec. 31, 2011 WPL [Member] Redeemable Preferred Stock [Member]	Dec. 31, 2012 WPL [Member] Redeemable Preferred Stock [Member] 4.50% [Member]		Dec. 31, 2011 WPL [Member] Redeemable Preferred Stock [Member] 4.50% [Member]		Dec. 31, 2012 WPL [Member] Redeemable Preferred Stock [Member] 4.80% [Member]		Dec. 31, 2011 WPL [Member] Redeemable Preferred Stock [Member] 4.80% [Member]		Dec. 31, 2012 WPL [Member] Redeemable Preferred Stock [Member] 4.96% [Member]		Dec. 31, 2011 WPL [Member] Redeemable Preferred Stock [Member] 4.96% [Member]		Dec. 31, 2012 WPL [Member] Redeemable Preferred Stock [Member] 4.40% [Member]		Dec. 31, 2011 WPL [Member] Redeemable Preferred Stock [Member] 4.40% [Member]		Dec. 31, 2012 WPL [Member] Redeemable Preferred Stock [Member] 4.76% [Member]		Dec. 31, 2011 WPL [Member] Redeemable Preferred Stock [Member] 4.76% [Member]		Dec. 31, 2012 WPL [Member] Redeemable Preferred Stock [Member] 6.20% [Member]		Dec. 31, 2011 WPL [Member] Redeemable Preferred Stock [Member] 6.20% [Member]		Dec. 31, 2012 WPL [Member] Redeemable Preferred Stock [Member] 6.50% [Member]		Dec. 31, 2011 WPL [Member] Redeemable Preferred Stock [Member] 6.50% [Member]		Mar. 31, 2013 WPL [Member] Redeemable Preferred Stock [Member] 4.40% through 6.50% [Member] Subsequent Event [Member] Scenario, Forecast [Member]	Mar. 31, 2013 Alliant Energy And IPL [Member] Redeemable Preferred Stock [Member] 8.375% [Member] Subsequent Event [Member] Scenario, Forecast [Member]	Mar. 31, 2013 Alliant Energy and WPL [Member] Redeemable Preferred Stock [Member] 4.40% through 6.50% [Member] Subsequent Event [Member] Scenario, Forecast [Member]
Temporary Equity [Line Items]
Liquidation Preference/Stated Value (in dollars per share)											$ 25
Liquidation Preference/Stated Value (in dollars per share)																			$ 100	[1]			$ 100	[1]			$ 100	[1]			$ 100	[1]			$ 100	[1]			$ 100	[1]			$ 25	[1]
Authorized Shares											16,000,000
Authorized Shares																			3,750,000				3,750,000				3,750,000				3,750,000				3,750,000				3,750,000				3,750,000
Shares Outstanding											6,000,000
Shares Outstanding																			99,970	[1]			74,912	[1]			64,979	[1]			29,957	[1]			29,947	[1]			150,000	[1]			599,460	[1]
Series						8.375%					8.375%			4.50%					4.50%	[1]			4.80%	[1]			4.96%	[1]			4.40%	[1]			4.76%	[1]			6.20%	[1]			6.50%	[1]
Redemption (none are mandatorily redeemable)											On or after March 15, 2013								Any time	[1]			Any time	[1]			Any time	[1]			Any time	[1]			Any time	[1]			Any time	[1]			Any time	[1]
Cumulative preferred stock value, at par	$ 145.1	$ 145.1				$ 145.1	$ 145.1		$ 145.1	$ 145.1	$ 150.0	$ 150.0
Less: discount									(4.9)	(4.9)
Cumulative preferred stock	60.0	60.0												60.0	60.0		60.0	60.0	10.0	[1]	10.0	[1]	7.5	[1]	7.5	[1]	6.5	[1]	6.5	[1]	3.0	[1]	3.0	[1]	3.0	[1]	3.0	[1]	15.0	[1]	15.0	[1]	15.0	[1]	15.0	[1]
Preferred stock and temporary equity net carrying amount				205.1	205.1
Preferred stock called during period (in shares)													6,000,000																																		1,049,225
Stock redeemed during period, value													150																																		61
Preferred dividends	$ 15.9	$ 18.3	$ 18.7			$ 12.6	$ 15.0	$ 15.4						$ 3.3	$ 3.3	$ 3.3																																$ 5.0	$ 1.0

[1] WPL has 3,750,000 authorized shares in total.