Investments - Realized Investment Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Gain (Loss) on Securities [Line Items]
Net investment losses	$ (9.0)	$ (18.5)
Fixed maturity securities
Gain (Loss) on Securities [Line Items]
Net investment losses	(5.4)	1.1
Equity securities
Gain (Loss) on Securities [Line Items]
Net investment losses	(2.7)	(14.7)
Short-term investments and other
Gain (Loss) on Securities [Line Items]
Net investment losses	$ (0.9)	$ (4.9)