Debt and Credit Arrangements - Interest Expense and Financing Cost Amortization (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Line of Credit Facility
Interest expense, debt	$ 18.1	$ 17.7	$ 15.9
Financing costs amortization	4.3	3.0	1.3
Revolving Credit Facility
Line of Credit Facility
Interest expense, debt	7.0	5.3	11.8
Financing costs amortization	3.6	2.0	0.6
Revolving Credit Facility | Interest expense, term loan due June 2024
Line of Credit Facility
Interest expense, debt	4.8	3.1	0.0
Revolving Credit Facility | Interest expense, senior secured revolving credit facilities
Line of Credit Facility
Interest expense, debt	$ 2.2	$ 2.2	$ 11.8