Investments (Unconsolidated Equity Investments) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Carrying value	$ 458.3	$ 421.3	$ 381.4
Equity (income) / loss	(53.0)	(54.6)	(44.8)
ATC Holdings [Member]
Schedule of Equity Method Investments [Line Items]
Carrying value	320.1	293.6
Equity (income) / loss	$ (45.3)	(38.1)	(42.4)
ATC LLC [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest	16.00%
ATC Holdco LLC [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest	20.00%
Non-utility wind farm in Oklahoma [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest	50.00%
Carrying value	$ 105.3	105.1
Equity (income) / loss	(5.2)	(15.6)	(1.8)
Other [Member]
Schedule of Equity Method Investments [Line Items]
Carrying value	32.9	22.6
Equity (income) / loss	(2.5)	(0.9)	(0.6)
Total [Member]
Schedule of Equity Method Investments [Line Items]
Carrying value	458.3	421.3
Equity (income) / loss	$ (53.0)	$ (54.6)	$ (44.8)