Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Statement of Financial Position [Abstract]
Fixed maturities available-for-sale, at cost	$ 4,145.2	$ 3,981.1
Allowance for expected credit losses	7.0	6.6
Equity securities, cost	131.1	162.5
Other investments, cost	425.8	429.4
Short-term investments, cost	$ 507.8	$ 541.4
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)	46,178,719	46,009,966
Treasury shares (in shares)	11,315,889	11,315,889