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Investments - Distribution of Fixed Maturity Securities by Expected Maturity (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2025	Dec. 31, 2024
Amortized Cost, net
Due in 1 year or less	$ 333.4
Due after 1 year through 5 years	1,340.3
Due after 5 years through 10 years	1,573.5
Due after 10 years through 20 years	1,634.5
Due after 20 years	1,103.8
Amortized Cost, net	5,985.5	$ 5,842.5
Fair Value
Due in 1 year or less	333.1
Due after 1 year through 5 years	1,323.0
Due after 5 years through 10 years	1,547.9
Due after 10 years through 20 years	1,509.8
Due after 20 years	952.7
Total	$ 5,666.5	$ 5,387.9
Percent of Total Fair Value
Due in 1 year Or less	5.90%
Due after 1 year through 5 years	23.30%
Due after 5 years through 10 years	27.30%
Due after 10 years through 20 years	26.60%
Due after 20 years	16.80%
Total	100.00%
Average option-adjusted duration, in years	5 years 10 months 24 days

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