Investments - Aging of Gross Unrealized Losses (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Fair Value
Less than 12 Months	$ 83.4	$ 168.6
12 Months or More	72.2	236.2
Fair Value	155.6	404.8
Gross Unrealized Losses
Less than 12 Months	(0.5)	(1.4)
12 Months or More	(0.8)	(4.9)
Gross Unrealized Losses	(1.3)	(6.3)
Corporate debt securities
Fair Value
Less than 12 Months	73.3	89.0
12 Months or More	63.9	178.3
Fair Value	137.2	267.3
Gross Unrealized Losses
Less than 12 Months	(0.5)	(1.2)
12 Months or More	(0.8)	(4.0)
Gross Unrealized Losses	(1.3)	(5.2)
U.S. Government obligations
Fair Value
Less than 12 Months	6.1	79.6
12 Months or More	8.3	57.7
Fair Value	14.4	137.3
Gross Unrealized Losses
Less than 12 Months	0.0	(0.2)
12 Months or More	0.0	(0.9)
Gross Unrealized Losses	0.0	(1.1)
Asset-backed securities
Fair Value
Less than 12 Months	4.0	0.0
12 Months or More	0.0	0.2
Fair Value	4.0	0.2
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