Investments (Unconsolidated Equity Investments) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Jun. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Schedule of Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	$ (9.8)	$ (10.1)		$ (41.6)	$ (32.9)
ATC Holdings [Member]
Schedule of Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	(8.9)	(10.1)		(25.4)	(32.7)
Non-utility wind farm In Oklahoma [Member]
Schedule of Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	0.1	0.2	$ 0.0	(14.5)	0.2
Other [Member]
Schedule of Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	$ (1.0)	$ (0.2)		$ (1.7)	$ (0.4)