Investments - Maturities of Fixed Maturities (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Dec. 31, 2025
Amortized Cost, net
Due in 1 year or less	$ 294.9
Due after 1 year through 5 years	1,302.1
Due after 5 years through 10 years	1,595.5
Due after 10 years through 20 years	1,688.6
Due after 20 years	1,266.3
Amortized Cost, net	6,147.4	$ 6,026.7
Fair Value
Due in 1 year or less	293.4
Due after 1 year through 5 years	1,282.2
Due after 5 years through 10 years	1,552.1
Due after 10 years through 20 years	1,545.9
Due after 20 years	1,106.5
Total	$ 5,780.1	$ 5,714.6
Percent of Total Fair Value
Due in 1 year Or less	5.10%
Due after 1 year through 5 years	22.20%
Due after 5 years through 10 years	26.90%
Due after 10 years through 20 years	26.70%
Due after 20 years	19.10%
Total	100.00%
Average option-adjusted duration, in years	6 years