Schedule II. Condensed Financial Information of Registrant (Condensed Balance Sheets Parenthetical) (Details) - USD ($) $ / shares in Units, $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Fixed maturity securities, amortized cost	$ 2,266.1	$ 2,333.6
Equity securities, cost	$ 212.6	$ 112.4
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	150,000,000	150,000,000
Common stock, shares issued (in shares)	57,690,254	57,413,806
Common stock, shares outstanding (in shares)	27,741,400	28,564,798
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, shares authorized (in shares)	25,000,000	25,000,000
Preferred stock, shares issued (in shares)	0	0
Treasury stock, at cost (in shares)	29,948,854	28,849,008
Parent Company [Member]
Fixed maturity securities, amortized cost	$ 9.1	$ 9.6
Equity securities, cost	25.2	0.4
Short-term Investments [Member]
Fixed maturity securities, amortized cost	$ 10.5	$ 26.5