Investments - Net Investment Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net Investment Income [Line Items]
Net investment income	$ 1.5	$ 3.4	$ 1.3
Interest on cash and cash equivalents
Net Investment Income [Line Items]
Net investment income	1.0	2.8	1.0
Bonds
Net Investment Income [Line Items]
Net investment income	0.1	0.1	0.1
Short-term investments
Net Investment Income [Line Items]
Net investment income	$ 0.4	$ 0.5	$ 0.2