Investments (Unconsolidated Equity Investments) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Schedule of Equity Method Investments [Line Items]
Equity income from unconsolidated investments, net	$ (10.6)	$ (9.6)	$ (20.0)	$ (19.5)
Alliant Energy [Member]
Schedule of Equity Method Investments [Line Items]
Equity income from unconsolidated investments, net	(10.6)	(9.6)	(20.0)	(19.5)
WPL [Member]
Schedule of Equity Method Investments [Line Items]
Equity income from unconsolidated investments, net	(10.6)	(9.7)	(20.7)	(19.1)
American Transmission Company LLC (ATC) [Member] | Alliant Energy [Member]
Schedule of Equity Method Investments [Line Items]
Equity income from unconsolidated investments, net	(10.3)	(9.3)	(20.2)	(18.5)
American Transmission Company LLC (ATC) [Member] | WPL [Member]
Schedule of Equity Method Investments [Line Items]
Equity income from unconsolidated investments, net	(10.3)	(9.3)	(20.2)	(18.5)
Other [Member] | Alliant Energy [Member]
Schedule of Equity Method Investments [Line Items]
Equity income from unconsolidated investments, net	(0.3)	(0.3)	0.2	(1.0)
Other [Member] | WPL [Member]
Schedule of Equity Method Investments [Line Items]
Equity income from unconsolidated investments, net	$ (0.3)	$ (0.4)	$ (0.5)	$ (0.6)