Leases - Lease Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Components of lease cost
Amortization of right-of-use assets	$ 0.7	$ 0.3
Interest on lease liabilities	0.1
Total finance lease cost	0.8	0.3
Operating lease cost	23.3	24.9
Short term lease cost	3.9	2.2
Variable lease cost	3.4	3.5
Sublease income	(1.9)	(1.2)
Total lease cost	$ 29.5	$ 29.7
Operating lease rental expense			$ 25.1