Significant Accounting Policies (Gain on Asset Dispositions) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Property, Plant and Equipment [Abstract]
Disposition gains	$ 49.7	$ 90.3	$ 63.1
Residual sharing income	83.6	13.4	9.4
Non-remarketing disposition gains	3.2	9.4	16.0
Asset impairments	38.5	33.9	1.3
Net Gain on Asset Dispositions	$ 98.0	$ 79.2	$ 87.2