Significant Accounting Policies (Gain on Asset Dispositions) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Abstract]
Disposition gains	$ 64.8	$ 44.1	$ 49.7
Residual sharing income	(2.5)	(10.2)	(83.6)
Non-remarketing disposition gains	(14.5)	(8.4)	(3.2)
Asset impairments	(9.0)	(8.6)	(38.5)
Net Gain on Asset Dispositions	$ 72.8	$ 54.1	$ 98.0