Investments (Unconsolidated Equity Investments) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Schedule of Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	$ (10.5)	$ (11.3)	$ (31.8)	$ (22.8)
ATC Holdings [Member]
Schedule of Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	(7.8)	(11.1)	(16.5)	(22.6)
Non-utility wind farm In Oklahoma [Member]
Schedule of Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	(2.5)	0.0	(14.6)	0.0
Other [Member]
Schedule of Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	$ (0.2)	$ (0.2)	$ (0.7)	$ (0.2)