Significant Accounting Policies (Gain on Asset Dispositions) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Abstract]
Disposition gains	$ 39.4	$ 58.6	$ 64.7
Residual sharing income	(2.5)	(1.9)	(2.5)
Non-remarketing disposition gains	(0.1)	2.3	(14.5)
Asset impairments	(0.3)	(6.6)	(9.0)
Net Gain on Asset Dispositions	$ 41.7	$ 51.6	$ 72.7