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Investments - Distribution of Fixed Maturity Securities by Expected Maturity (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Dec. 31, 2024
Amortized Cost, net
Due in 1 year or less	$ 238.2
Due after 1 year through 5 years	1,406.8
Due after 5 years through 10 years	1,509.7
Due after 10 years through 20 years	1,550.9
Due after 20 years	1,152.1
Amortized Cost, net	5,857.7	$ 5,842.5
Fair Value
Due in 1 year or less	235.9
Due after 1 year through 5 years	1,383.0
Due after 5 years through 10 years	1,453.4
Due after 10 years through 20 years	1,405.6
Due after 20 years	979.8
Total	$ 5,457.7	$ 5,387.9
Percent of Total Fair Value
Due in 1 year Or less (percent)	4.30%
Due after 1 year through 5 years (percent)	25.30%
Due after 5 years through 10 years (percent)	26.60%
Due after 10 years through 20 years (percent)	25.80%
Due after 20 years (percent)	18.00%
Total (percent)	100.00%
Average option-adjusted duration, in years	5 years 6 months

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Investment debt securities average option adjusted duration expressed in number of years.

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