Investments - Aging of Gross Unrealized Losses (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Fair Value
Less than 12 Months	$ 182.8	$ 168.6
12 Months or More	53.0	236.2
Fair Value	235.8	404.8
Gross Unrealized Losses
Less than 12 Months	(0.8)	(1.4)
12 Months or More	(0.5)	(4.9)
Gross Unrealized Losses	(1.3)	(6.3)
Corporate debt securities
Fair Value
Less than 12 Months	149.4	89.0
12 Months or More	53.0	178.3
Fair Value	202.4	267.3
Gross Unrealized Losses
Less than 12 Months	(0.6)	(1.2)
12 Months or More	(0.5)	(4.0)
Gross Unrealized Losses	(1.1)	(5.2)
U.S. Government obligations
Fair Value
Less than 12 Months	27.6	79.6
12 Months or More	0.0	57.7
Fair Value	27.6	137.3
Gross Unrealized Losses
Less than 12 Months	(0.2)	(0.2)
12 Months or More	0.0	(0.9)
Gross Unrealized Losses	(0.2)	(1.1)
Asset-backed securities
Fair Value
Less than 12 Months	5.8	0.0
12 Months or More	0.0	0.2
Fair Value	5.8	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
Non-U.S. government obligations
Fair Value
Less than 12 Months	0.0	0.0
12 Months or More	0.0	0.0
Fair Value	0.0	0.0
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