Investments (Unconsolidated Equity Investments) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Schedule of Equity Method Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	$ (11.1)	$ (10.4)	$ (32.7)	$ (30.4)
WPL [Member]
Schedule of Equity Method Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	(11.1)	(10.5)	(32.7)	(31.2)
ATC [Member]
Schedule of Equity Method Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	(10.8)	(10.4)	(31.7)	(30.6)
ATC [Member] | WPL [Member]
Schedule of Equity Method Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	(10.8)	(10.4)	(31.7)	(30.6)
Other [Member]
Schedule of Equity Method Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	(0.3)	0	(1.0)	0.2
Other [Member] | WPL [Member]
Schedule of Equity Method Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	(0.3)	(0.1)	(1.0)	(0.6)
Totals [Member]
Schedule of Equity Method Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	(11.1)	(10.4)	(32.7)	(30.4)
Totals [Member] | WPL [Member]
Schedule of Equity Method Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	$ (11.1)	$ (10.5)	$ (32.7)	$ (31.2)