Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2022	Dec. 31, 2021
Statement of Financial Position [Abstract]
Fixed maturities available-for-sale, at cost	$ 4,131.5	$ 4,203.2
Allowance for expected credit losses	3.3	2.5
Cost	158.8
Allowance for credit loss	0.2
Equity securities, cost	55.3	70.3
Other investments, cost	401.6	387.0
Short-term investments, cost	$ 570.2	$ 655.4
Preferred shares, par value (in dollars per share)	$ 1.00	$ 1.00
Preferred shares, shares authorized (in shares)	30,000,000	30,000,000
Preferred shares, shares issued (in shares)	6,000	6,000
Liquidation preference (in dollars per share)	$ 25,000	$ 25,000
Common shares, par value (in dollars per share)	$ 1.00	$ 1.00
Common shares, shares authorized (in shares)	500,000,000	500,000,000
Common shares, shares issued (in shares)	46,339,696	46,192,867
Treasury shares (in shares)	11,318,339	11,318,339