Leases - Lease cost components (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Lease cost components
Amortization of leased asset	$ 5.3
Interest on lease liabilities	0.6
Operating lease cost	9.8
Net lease cost	15.7
Short-term lease and variable lease costs	$ 1.6
Rental expense under operating lease		$ 9.2	$ 8.8