Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Statement of Financial Position [Abstract]
Fixed maturities available-for-sale, cost	$ 3,016.4	$ 4,203.2
Fixed maturities available-for-sale, allowance for expected credit losses	2.8	2.5
Commercial mortgage loans, cost	159.9
Commercial mortgage loans, allowance for expected credit losses	0.2
Equity securities, cost	54.7	70.3
Other investments, cost	323.2	387.0
Short-term investments, cost	$ 449.4	$ 655.4
Preferred shares, par value (in dollars per share)	$ 1.00	$ 1.00
Preferred shares, authorized (in shares)	30,000,000	30,000,000
Preferred shares, issued (in shares)	6,000	6,000
Preferred stock, 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)	46,379,297	46,192,867
Treasury shares (in shares)	11,318,339	11,315,889