Investments - Schedule of Continuous Loss Position (Details) - USD ($) $ in Millions	Jun. 30, 2026	Dec. 31, 2025
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Less than 12 months, Fair Value	$ 363.4	$ 88.6
12 months or longer, Fair Value	5.3	16.3
Total Fair Value	368.7	104.9
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Less than 12 months, Gross Unrealized Losses	(3.4)	(0.4)
12 months or longer, Gross Unrealized Losses	(0.2)	(0.7)
Total Gross Unrealized Losses	(3.6)	(1.1)
Corporate
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Less than 12 months, Fair Value	135.5	25.6
12 months or longer, Fair Value	0.0	0.0
Total Fair Value	135.5	25.6
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Less than 12 months, Gross Unrealized Losses	(1.1)	(0.1)
12 months or longer, Gross Unrealized Losses		0.0
Total Gross Unrealized Losses	(1.1)	(0.1)
US government and agency
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Less than 12 months, Fair Value	45.3	0.0
12 months or longer, Fair Value	0.0	4.8
Total Fair Value	45.3	4.8
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Less than 12 months, Gross Unrealized Losses	(0.5)	0.0
12 months or longer, Gross Unrealized Losses	0.0	(0.1)
Total Gross Unrealized Losses	(0.5)	(0.1)
Non-US government and agency
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Less than 12 months, Fair Value	145.1	63.0
12 months or longer, Fair Value	5.3	8.1
Total Fair Value	150.4	71.1
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Less than 12 months, Gross Unrealized Losses	(1.4)	(0.3)
12 months or longer, Gross Unrealized Losses	(0.2)	(0.1)
Total Gross Unrealized Losses	(1.6)	(0.4)
Residential mortgage-backed
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Less than 12 months, Fair Value	29.4	0.0
12 months or longer, Fair Value	0.0	3.4
Total Fair Value	29.4	3.4
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Less than 12 months, Gross Unrealized Losses	(0.3)	0.0
12 months or longer, Gross Unrealized Losses	0.0	(0.5)
Total Gross Unrealized Losses	(0.3)	$ (0.5)
Other asset-backed securities
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Less than 12 months, Fair Value	8.1
12 months or longer, Fair Value	0.0
Total Fair Value	8.1
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Less than 12 months, Gross Unrealized Losses	(0.1)
12 months or longer, Gross Unrealized Losses	0.0
Total Gross Unrealized Losses	$ (0.1)

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