Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Net Investment Income [Line Items]
Net investment income	$ 0.2	$ 0.5	$ 1.1	$ 1.0
Interest on cash and cash equivalents
Net Investment Income [Line Items]
Net investment income	0.1	0.4	0.8	0.8
Short-term investments
Net Investment Income [Line Items]
Net investment income	$ 0.1	$ 0.1	$ 0.3	$ 0.2