Investments (Unconsolidated Equity Investments) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Schedule of Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	$ (10.1)	$ (9.2)	$ (32.9)	$ (28.8)
WPL [Member]
Schedule of Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	(0.2)	(9.3)	(0.4)	(29.0)
ATC Investment [Member]
Schedule of Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	(10.1)	(9.1)	(32.7)	(28.6)
ATC Investment [Member] | WPL [Member]
Schedule of Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	0.0	(9.1)	0.0	(28.6)
Other [Member]
Schedule of Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	0.0	(0.1)	(0.2)	(0.2)
Other [Member] | WPL [Member]
Schedule of Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	$ (0.2)	$ (0.2)	$ (0.4)	$ (0.4)