Investments (Unconsolidated Equity Investments) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Schedule of Equity Method Investments [Line Items]
Equity income from unconsolidated investments, net	$ (10.4)	$ (10.1)	$ (30.4)	$ (29.6)
Alliant Energy [Member]
Schedule of Equity Method Investments [Line Items]
Equity income from unconsolidated investments, net	(10.4)	(10.1)	(30.4)	(29.6)
WPL [Member]
Schedule of Equity Method Investments [Line Items]
Equity income from unconsolidated investments, net	(10.5)	(9.8)	(31.2)	(28.9)
American Transmission Company LLC (ATC) [Member] | Alliant Energy [Member]
Schedule of Equity Method Investments [Line Items]
Equity income from unconsolidated investments, net	(10.4)	(9.7)	(30.6)	(28.2)
American Transmission Company LLC (ATC) [Member] | WPL [Member]
Schedule of Equity Method Investments [Line Items]
Equity income from unconsolidated investments, net	(10.4)	(9.7)	(30.6)	(28.2)
Other [Member] | Alliant Energy [Member]
Schedule of Equity Method Investments [Line Items]
Equity income from unconsolidated investments, net	0	(0.4)	0.2	(1.4)
Other [Member] | WPL [Member]
Schedule of Equity Method Investments [Line Items]
Equity income from unconsolidated investments, net	$ (0.1)	$ (0.1)	$ (0.6)	$ (0.7)