Income Taxes (Rollforward of Uncertain Tax Positions) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Uncertain Tax Positions [Rollforward]
Uncertain tax positions, beginning balance	$ 0.0	$ 0.0	$ 0.0
Additions based on tax positions related to the current year	3.2	0.0	0.0
Additions for tax positions of prior years	4.9	0.0	0.0
Uncertain tax positions, ending balance	$ 8.1	$ 0.0	$ 0.0