Investments - Distribution of Fixed Maturity Securities by Expected Maturity (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Amortized Cost, net
Due in 1 year or less	$ 296.6
Due after 1 year through 5 years	1,497.3
Due after 5 years through 10 years	1,524.5
Due after 10 years through 20 years	1,452.8
Due after 20 years	1,045.3
Amortized Cost, net	5,816.5	$ 5,652.9
Fair Value
Due in 1 year or less	292.4
Due after 1 year through 5 years	1,451.5
Due after 5 years through 10 years	1,449.0
Due after 10 years through 20 years	1,289.1
Due after 20 years	863.4
Total	$ 5,345.4	$ 5,235.3
Percent of Total Fair Value
Due in 1 year Or less (percent)	5.50%
Due after 1 year through 5 years (percent)	27.10%
Due after 5 years through 10 years (percent)	27.10%
Due after 10 years through 20 years (percent)	24.10%
Due after 20 years (percent)	16.20%
Total (percent)	100.00%
Average option-adjusted duration, in years	5 years 9 months 18 days