Investments - Realized Investment (Losses) Gains (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Gain (Loss) on Securities
Net investment (losses) gains	$ (6.5)	$ 2.5	$ (10.6)	$ (12.8)
Fixed maturity securities
Gain (Loss) on Securities
Net investment (losses) gains	(4.0)	2.7	(7.9)	3.1
Equity securities
Gain (Loss) on Securities
Net investment (losses) gains	(1.0)	4.0	0.7	(3.6)
Short-term investments and other
Gain (Loss) on Securities
Net investment (losses) gains	$ (1.5)	$ (4.2)	$ (3.4)	$ (12.3)