Debt, Interest Income, Expense and Other Finance Costs - Interest Income (Expense), and Other Financing Costs, net (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Debt, Interest Income, Expense and Other Finance Costs [Abstract]
Interest income	$ 1.3	$ 1.7	$ 1.9	$ 2.7
Interest expense and other financing costs	(19.2)	(15.5)	(36.1)	(29.2)
Interest income (expense) and other financing costs, net	$ (17.9)	$ (13.8)	$ (34.2)	$ (26.5)