Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Statement of Financial Position [Abstract]
Fixed maturities available-for-sale, cost	$ 3,847.0	$ 3,605.0
Allowance for expected credit losses	33.7	0.0
Equity securities, cost	160.6	122.8
Other investments, cost	412.1	482.5
Short-term investments, cost	$ 528.5	$ 844.8
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	0
Liquidation preference (in dollars per share)	$ 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)	45,985,685	45,698,470
Treasury shares (in shares)	11,315,889	11,315,889