Leases - Lease cost components (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Lease cost components
Amortization of leased asset	$ 10.7	$ 5.3
Interest on lease liabilities	1.2	0.6
Operating lease cost	10.0	9.8
Net lease cost	21.9	15.7
Short-term lease and variable lease costs	$ 1.6	$ 1.6
Rental expense under operating lease			$ 9.2