Investments - Maturities of Fixed Maturities (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Amortized Cost, net
Due in 1 year or less	$ 298.6
Due after 1 year through 5 years	1,482.4
Due after 5 years through 10 years	1,435.1
Due after 10 years through 20 years	1,426.3
Due after 20 years	1,010.5
Amortized Cost, net	5,652.9	$ 5,756.9
Fair Value
Due in 1 year or less	294.1
Due after 1 year through 5 years	1,435.5
Due after 5 years through 10 years	1,371.5
Due after 10 years through 20 years	1,282.6
Due after 20 years	851.6
Total	$ 5,235.3	$ 5,185.0	[1]
Percent of Total Fair Value
Due in 1 year or less	5.60%
Due after 1 year through 5 years	27.40%
Due after 5 years through 10 years	26.20%
Due after 10 years through 20 years	24.50%
Due after 20 years	16.30%
Total	100.00%

[1] Recast for the adoption of ASU 2018-12. See Note 1 of the Consolidated Financial Statements