Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Fixed maturities available-for-sale, cost	$ 3,580.1	$ 3,529.1
Equity securities, cost	305.2	310.6
Other investments, cost	473.7	482.0
Short-term investments, cost	$ 444.1	$ 482.3
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)	45,365,223	45,276,999
Treasury shares (in shares)	11,315,889	11,315,889