Investments - Distribution of Fixed Maturity Securities by Expected Maturity (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2021	Dec. 31, 2020
Percent of Total Fair Value
Due in 1 year or less	3.90%	4.00%
Due after 1 year through 5 years	26.60%	28.30%
Due after 5 years through 10 years	28.40%	28.00%
Due after 10 years through 20 years	24.30%	24.60%
Due after 20 years	16.80%	15.10%
Total	100.00%	100.00%
Fair Value
Due in 1 year or less	$ 254.9
Due after 1 year through 5 years	1,745.1
Due after 5 years through 10 years	1,861.9
Due after 10 years through 20 years	1,595.1
Due after 20 years	1,098.0
Total	6,555.0	$ 6,345.3
Amortized Cost, net
Due in 1 year or less	247.8
Due after 1 year through 5 years	1,655.2
Due after 5 years through 10 years	1,702.2
Due after 10 years through 20 years	1,433.7
Due after 20 years	1,010.5
Amortized Cost, net	$ 6,049.4	$ 5,788.6
Average option-adjusted duration, in years	6 years 9 months 18 days	6 years 4 months 24 days