Investments (Unconsolidated Equity Investments) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Schedule of Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	$ (10.5)	$ (6.5)
WPL [Member]
Schedule of Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	(10.7)	(7.8)
ATC [Member]
Schedule of Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	(10.7)	(7.8)
ATC [Member] | WPL [Member]
Schedule of Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	(10.7)	(7.8)
Other [Member]
Schedule of Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	0.2	1.3
Other [Member] | WPL [Member]
Schedule of Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	$ 0.0	$ 0.0