Investments (Unconsolidated Equity Investments) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Schedule of Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	$ (11.5)	$ (11.1)	$ (34.2)	$ (32.7)
WPL [Member]
Schedule of Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	(11.4)	(11.1)	(34.2)	(32.7)
ATC [Member]
Schedule of Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	(11.2)	(10.8)	(33.5)	(31.7)
ATC [Member] | WPL [Member]
Schedule of Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	(11.2)	(10.8)	(33.5)	(31.7)
Other [Member]
Schedule of Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	(0.3)	(0.3)	(0.7)	(1.0)
Other [Member] | WPL [Member]
Schedule of Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	$ (0.2)	$ (0.3)	$ (0.7)	$ (1.0)