Investments - Maturities of Fixed Maturities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Amortized Cost, net
Due in 1 year or less	$ 245.4
Due after 1 year through 5 years	1,613.8
Due after 5 years through 10 years	1,600.5
Due after 10 years through 20 years	1,339.9
Due after 20 years	998.1
Amortized Cost, net	5,797.7	$ 5,788.6
Fair Value
Due in 1 year or less	250.8
Due after 1 year through 5 years	1,686.2
Due after 5 years through 10 years	1,729.5
Due after 10 years through 20 years	1,488.2
Due after 20 years	1,084.6
Total	$ 6,239.3	$ 6,345.3
Percent of Total Fair Value
Due in 1 year or less	4.00%
Due after 1 year through 5 years	27.00%
Due after 5 years through 10 years	27.70%
Due after 10 years through 20 years	23.90%
Due after 20 years	17.40%
Total	100.00%
Average option-adjusted duration, in years	6 years 8 months 12 days