Investments (Unconsolidated Equity Investments) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Schedule of Equity Method Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	$ (13.4)	$ (10.9)
ATC Holdings [Member]
Schedule of Equity Method Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	(11.0)	(9.5)
Non-utility wind farm in Oklahoma [Member]
Schedule of Equity Method Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	(2.2)	(1.1)
Other [Member]
Schedule of Equity Method Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	$ (0.2)	$ (0.3)