Investments - Distribution of Fixed Maturity Securities by Expected Maturity (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Percent of Total Fair Value
Due in 1 year or less	4.10%	4.00%
Due after 1 year through 5 years	28.20%	28.30%
Due after 5 years through 10 years	28.30%	28.00%
Due after 10 years through 20 years	24.00%	24.60%
Due after 20 years	15.40%	15.10%
Total	100.00%	100.00%
Fair Value
Due in 1 year or less	$ 261.3
Due after 1 year through 5 years	1,789.8
Due after 5 years through 10 years	1,799.7
Due after 10 years through 20 years	1,523.5
Due after 20 years	982.0
Total	6,356.3	$ 6,345.3
Amortized Cost, net
Due in 1 year or less	255.5
Due after 1 year through 5 years	1,713.5
Due after 5 years through 10 years	1,669.5
Due after 10 years through 20 years	1,412.2
Due after 20 years	942.0
Amortized Cost, net	$ 5,992.7	$ 5,788.6
Average option-adjusted duration, in years	6 years 7 months 6 days	6 years 4 months 24 days