Supplemental Information (Components of Interest Expense) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Summary of components of interest expense
Interest expense, net of capitalized interest	$ 32.1	$ 28.1	$ 25.2
Interest income	1.5	1.1	1.6
Interest expense, net	$ 30.6	$ 27.0	$ 23.6