Supplemental Information (Components of Interest Expense) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Summary of components of interest expense
Interest expense, net of capitalized interest	$ 29.7	$ 48.6	$ 39.1
Less: Interest income	1.4	1.1	0.8
Interest expense, net	$ 28.3	$ 47.5	$ 38.3