Leases - Lease cost (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Lease cost:
Operating lease cost	$ 0.5	$ 0.4	$ 1.1	$ 0.9
Short-term lease cost		0.3		0.9
Sublease income	(0.3)	(0.3)	(0.6)	(0.6)
Total lease cost	$ 0.2	$ 0.4	$ 0.5	$ 1.2