Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Statement of Financial Position [Abstract]
Fixed maturities available-for-sale, cost	$ 3,634.4	$ 3,605.0
Allowance for Credit Losses	31.6	0.0
Equity securities, cost	170.6	122.8
Other investments, cost	422.5	482.5
Short-term investments, cost	$ 732.7	$ 844.8
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)	45,867,812	45,698,470
Treasury shares (in shares)	11,315,889	11,315,889