Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Statement of Financial Position [Abstract]
Fixed maturities available-for-sale, cost	$ 2,521.5	$ 3,016.4
Fixed maturities available-for-sale, allowance for expected credit losses	0.2	2.8
Commercial mortgage loans, cost	144.8	159.9
Commercial mortgage loans, allowance for expected credit losses	0.2	0.2
Equity securities, cost	11.7	54.7
Other investments, cost	311.0	323.2
Short-term investments, cost	$ 429.0	$ 449.4
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)	2,000,000,000	500,000,000
Common shares, shares issued (in shares)	1,056,638,730	46,379,297
Treasury shares (in shares)	0	11,318,339