Investments (Unconsolidated Equity Investments) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Schedule of Equity Method Investments [Line Items]
Equity income from unconsolidated investments, net	$ (10.7)	$ (9.4)
WPL [Member]
Schedule of Equity Method Investments [Line Items]
Equity income from unconsolidated investments, net	(10.8)	(10.1)
American Transmission Company LLC (ATC) [Member]
Schedule of Equity Method Investments [Line Items]
Equity income from unconsolidated investments, net	(10.3)	(9.9)
American Transmission Company LLC (ATC) [Member] | WPL [Member]
Schedule of Equity Method Investments [Line Items]
Equity income from unconsolidated investments, net	(10.3)	(9.9)
Other [Member]
Schedule of Equity Method Investments [Line Items]
Equity income from unconsolidated investments, net	(0.4)	0.5
Other [Member] | WPL [Member]
Schedule of Equity Method Investments [Line Items]
Equity income from unconsolidated investments, net	(0.5)	(0.2)
Totals [Member]
Schedule of Equity Method Investments [Line Items]
Equity income from unconsolidated investments, net	(10.7)	(9.4)
Totals [Member] | WPL [Member]
Schedule of Equity Method Investments [Line Items]
Equity income from unconsolidated investments, net	$ (10.8)	$ (10.1)