Investments - Distribution of Fixed Maturity Securities by Expected Maturity (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2023	Dec. 31, 2022
Percent of Total Fair Value
Due in 1 year Or less (percent)	4.90%	4.40%
Due after 1 year through 5 years (percent)	26.70%	26.30%
Due after 5 years through 10 years (percent)	27.50%	27.90%
Due after 10 years through 20 years (percent)	24.60%	25.00%
Due after 20 years (percent)	16.30%	16.40%
Total (percent)	100.00%	100.00%
Fair Value
Due in 1 year or less	$ 253.5
Due after 1 year through 5 years	1,390.7
Due after 5 years through 10 years	1,431.8
Due after 10 years through 20 years	1,286.9
Due after 20 years	849.0
Total	5,211.9	$ 5,185.0
Amortized Cost, net
Due in 1 year or less	261.6
Due after 1 year through 5 years	1,448.0
Due after 5 years through 10 years	1,520.5
Due after 10 years through 20 years	1,444.5
Due after 20 years	1,038.1
Amortized Cost, net	$ 5,712.7	$ 5,756.9
Average option-adjusted duration, in years	6 years 2 months 12 days	6 years 4 months 24 days