Investments (Unconsolidated Equity Investments) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Schedule of Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	$ (11.1)	$ (11.5)	$ (28.9)	$ (34.2)
WPL [Member]
Schedule of Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	(11.1)	(11.4)	(30.2)	(34.2)
ATC [Member]
Schedule of Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	(10.9)	(11.2)	(29.6)	(33.5)
ATC [Member] | WPL [Member]
Schedule of Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	(10.9)	(11.2)	(29.6)	(33.5)
Other [Member]
Schedule of Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	(0.2)	(0.3)	0.7	(0.7)
Other [Member] | WPL [Member]
Schedule of Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	$ (0.2)	$ (0.2)	$ (0.6)	$ (0.7)