Other Income, net (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other Income, net
Agency and commitment fees	$ (3.1)	$ (3.0)	$ (2.0)
Interest income	20.2	11.5	18.4
Other income, net	$ 17.1	$ 8.5	$ 16.4