Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Fixed maturities available-for-sale, cost	$ 3,496.5	$ 3,320.6
Equity securities, cost	358.6	338.2
Other investments, cost	477.8	534.1
Short-term investments, cost	$ 434.5	$ 368.5
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)	45,188,722	40,385,309
Treasury shares (in shares)	11,285,996	10,785,007