Debt, Interest Income, Expense and Other Finance Costs - Interest Income (Expense), and Other Financing Costs, net (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Debt, Interest Income, Expense and Other Finance Costs [Abstract]
Interest income	$ 0.8	$ 1.2	$ 2.7	$ 3.9
Interest expense and other financing costs	(19.1)	(17.0)	(55.1)	(46.2)
Interest income (expense) and other financing costs, net	$ (18.3)	$ (15.8)	$ (52.5)	$ (42.2)