Debt and Credit Arrangements - Interest and Financing Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Line of Credit Facility
Interest expense, debt	$ 15.9	$ 9.3	$ 5.6
Financing costs amortization	1.3	1.3	1.3
Revolving Credit Facility | Senior Secured Revolving Credit Facility, due 2022
Line of Credit Facility
Interest expense, debt	11.8	2.7	0.0
Financing costs amortization	$ 0.6	$ 0.6	$ 0.6	$ 0.6