Investments - Realized Investment Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Gain (Loss) on Securities [Line Items]
Net investment gains (losses)	$ 4.9	$ 3.2	$ (4.1)	$ (15.3)
Fixed maturity securities
Gain (Loss) on Securities [Line Items]
Net investment gains (losses)	1.5	(0.7)	(3.9)	0.4
Equity securities
Gain (Loss) on Securities [Line Items]
Net investment gains (losses)	4.4	7.1	1.7	(7.6)
Short-term investments and other
Gain (Loss) on Securities [Line Items]
Net investment gains (losses)	$ (1.0)	$ (3.2)	$ (1.9)	$ (8.1)