Leases - Lease Cost (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Finance lease cost:
Amortization of right-of-use assets	$ 0.2	$ 0.3	$ 0.8	$ 0.9
Interest on lease liabilities	0.0	0.0	0.0	0.1
Net finance lease cost	0.2	0.3	0.8	1.0
Operating lease cost	7.9	6.8	15.7	14.0
Variable lease cost	0.1	0.0	0.3	0.2
Total lease cost	$ 8.2	$ 7.1	$ 16.8	$ 15.2