Investments (Unconsolidated Equity Investments) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Investments in equity method subsidiaries	$ 249.6		$ 238.5		$ 229.2
Equity (income)/loss from unconsolidated investments, net	(39.3)		(38.1)		(36.6)
WPL [Member]
Investments in equity method subsidiaries	246.5		236.0		227.1
Equity (income)/loss from unconsolidated investments, net	(38.7)		(37.8)		(37.0)
American Transmission Company LLC (ATC) [Member]
Unconsolidated Equity Investments, ownership interest	16.00%	[1]
Investments in equity method subsidiaries	238.8	[1]	227.9	[1]
Equity (income)/loss from unconsolidated investments, net	(37.8)	[1]	(36.9)	[1]	(36.1)	[1]
American Transmission Company LLC (ATC) [Member] | WPL [Member]
Unconsolidated Equity Investments, ownership interest	16.00%	[1]
Investments in equity method subsidiaries	238.8	[1]	227.9	[1]
Equity (income)/loss from unconsolidated investments, net	(37.8)	[1]	(36.9)	[1]	(36.1)	[1]
Wisconsin River Power Company [Member]
Unconsolidated Equity Investments, ownership interest	50.00%
Investments in equity method subsidiaries	7.7		8.1
Equity (income)/loss from unconsolidated investments, net	(0.9)		(0.9)		(0.9)
Wisconsin River Power Company [Member] | WPL [Member]
Unconsolidated Equity Investments, ownership interest	50.00%
Investments in equity method subsidiaries	7.7		8.1
Equity (income)/loss from unconsolidated investments, net	(0.9)		(0.9)		(0.9)
Other Equity [Member]
Ownership Interest (Non-exact)	Various
Investments in equity method subsidiaries	3.1		2.5
Equity (income)/loss from unconsolidated investments, net	(0.6)		(0.3)		0.4
Totals [Member]
Investments in equity method subsidiaries	249.6		238.5
Equity (income)/loss from unconsolidated investments, net	(39.3)		(38.1)		(36.6)
Totals [Member] | WPL [Member]
Investments in equity method subsidiaries	246.5		236.0
Equity (income)/loss from unconsolidated investments, net	$ (38.7)		$ (37.8)		$ (37.0)

[1] Alliant Energy and WPL have the ability to exercise significant influence over ATC's financial and operating policies through their participation on ATC's Board of Directors. Refer to Note 21 for information regarding related party transactions with ATC.