Investments (Unconsolidated Equity Investments) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Schedule of Equity Method Investments [Line Items]
Equity income from unconsolidated investments, net	$ (9.4)	$ (9.9)
WPL [Member]
Schedule of Equity Method Investments [Line Items]
Equity income from unconsolidated investments, net	(10.1)	(9.4)
American Transmission Company LLC (ATC) [Member]
Schedule of Equity Method Investments [Line Items]
Equity income from unconsolidated investments, net	(9.9)	(9.2)
American Transmission Company LLC (ATC) [Member] | WPL [Member]
Schedule of Equity Method Investments [Line Items]
Equity income from unconsolidated investments, net	(9.9)	(9.2)
Other Equity [Member]
Schedule of Equity Method Investments [Line Items]
Equity income from unconsolidated investments, net	0.5	(0.7)
Other Equity [Member] | WPL [Member]
Schedule of Equity Method Investments [Line Items]
Equity income from unconsolidated investments, net	$ (0.2)	$ (0.2)