Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Statement of Financial Position [Abstract]
Fixed maturities available-for-sale, at cost	$ 2,370.0	$ 2,521.5
Allowance for expected credit losses	0.2	0.2
Mortgage loans	171.6	144.8
Allowance for credit loss	0.5	0.2
Equity securities, cost	9.5	11.7
Other investments, cost	344.4	311.0
Short-term investments, cost	$ 1,073.7	$ 429.0
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)	2,000,000,000	2,000,000,000
Common shares, shares issued (in shares)	1,156,638,730	1,056,638,730