Investments (Unconsolidated Equity Investments) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Schedule of Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	$ (21.3)	$ (11.5)
ATC Holdings [Member]
Schedule of Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	(8.7)	(11.5)
Non-utility wind farm In Oklahoma [Member]
Schedule of Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	(12.1)	0.0
Other [Member]
Schedule of Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	$ (0.5)	$ 0.0