Investments - Aging of Gross Unrealized Losses (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Fair Value
Less than 12 Months	$ 168.6	$ 116.0
12 Months or More	236.2	513.2
Fair Value	404.8	629.2
Gross Unrealized Losses
Less than 12 Months	(1.4)	(2.3)
12 Months or More	(4.9)	(21.0)
Gross Unrealized Losses	(6.3)	(23.3)
Corporate debt securities
Fair Value
Less than 12 Months	89.0	83.6
12 Months or More	178.3	428.1
Fair Value	267.3	511.7
Gross Unrealized Losses
Less than 12 Months	(1.2)	(2.1)
12 Months or More	(4.0)	(17.5)
Gross Unrealized Losses	(5.2)	(19.6)
U.S. Government obligations
Fair Value
Less than 12 Months	79.6	29.6
12 Months or More	57.7	85.1
Fair Value	137.3	114.7
Gross Unrealized Losses
Less than 12 Months	(0.2)	(0.2)
12 Months or More	(0.9)	(3.5)
Gross Unrealized Losses	(1.1)	(3.7)
Asset-backed securities
Fair Value
Less than 12 Months	0.0	2.8
12 Months or More	0.2	0.0
Fair Value	0.2	2.8
Gross Unrealized Losses
Less than 12 Months	0.0	0.0
12 Months or More	0.0	0.0
Gross Unrealized Losses	$ 0.0	$ 0.0