Investments (Unconsolidated Equity Investments) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Schedule of Equity Method Investments [Line Items]
Equity income from unconsolidated investments, net	$ (17.4)	$ (12.7)	$ (30.8)	$ (23.6)
ATC Holdings [Member]
Schedule of Equity Method Investments [Line Items]
Equity income from unconsolidated investments, net	(13.5)	(10.0)	(24.5)	(19.5)
Non-utility wind farm in Oklahoma [Member]
Schedule of Equity Method Investments [Line Items]
Equity income from unconsolidated investments, net	(1.9)	(1.3)	(4.1)	(2.4)
Other [Member]
Schedule of Equity Method Investments [Line Items]
Equity income from unconsolidated investments, net	$ (2.0)	$ (1.4)	$ (2.2)	$ (1.7)