Investments - Maturities of Fixed Maturities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021
Amortized Cost, net
Due in 1 year or less	$ 235.4
Due after 1 year through 5 years	1,423.3
Due after 5 years through 10 years	1,558.5
Due after 10 years through 20 years	1,477.9
Due after 20 years	1,061.8
Amortized Cost, net	5,756.9	$ 5,797.7
Fair Value
Due in 1 year or less	226.7
Due after 1 year through 5 years	1,362.4
Due after 5 years through 10 years	1,448.0
Due after 10 years through 20 years	1,296.2
Due after 20 years	851.7
Total	$ 5,185.0	$ 6,239.3
Percent of Total Fair Value
Due in 1 year or less	4.40%
Due after 1 year through 5 years	26.30%
Due after 5 years through 10 years	27.90%
Due after 10 years through 20 years	25.00%
Due after 20 years	16.40%
Total	100.00%
Average option-adjusted duration, in years	6 years 4 months 24 days