Investments - Distribution of Fixed Maturity Securities by Expected Maturity (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Percent of Total Fair Value
Due in 1 year or less	4.00%	4.00%
Due after 1 year through 5 years	26.20%	28.30%
Due after 5 years through 10 years	28.10%	28.00%
Due after 10 years through 20 years	23.90%	24.60%
Due after 20 years	17.80%	15.10%
Total	100.00%	100.00%
Fair Value
Due in 1 year or less	$ 260.5
Due after 1 year through 5 years	1,703.6
Due after 5 years through 10 years	1,830.4
Due after 10 years through 20 years	1,559.5
Due after 20 years	1,158.0
Total	6,512.0	$ 6,345.3
Amortized Cost, net
Due in 1 year or less	253.7
Due after 1 year through 5 years	1,620.1
Due after 5 years through 10 years	1,685.7
Due after 10 years through 20 years	1,411.2
Due after 20 years	1,074.9
Total	$ 6,045.6	$ 5,788.6
Average option-adjusted duration, in years	6 years 9 months 18 days	6 years 4 months 24 days