Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022
Statement of Financial Position [Abstract]
Fixed maturities available-for-sale, at cost	$ 2,969.8	$ 3,016.4
Allowance for expected credit losses	2.7	2.8
Cost	159.9	159.9
Allowance for credit loss	0.2	0.2
Equity securities, cost	48.7	54.7
Other investments, cost	323.0	323.2
Short-term investments, cost	$ 694.5	$ 449.4
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)	46,469,201	46,379,297
Treasury shares (in shares)	11,318,339	11,318,339