Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Available-for-sale cost	$ 3,529.1	$ 3,320.6
Equity securities cost	310.6	338.2
Other investments cost	482.0	534.1
Short-term investments, Cost	$ 482.3	$ 368.5
Common shares, par value	$ 1.00	$ 1.00
Common shares, shares authorized	500,000,000	500,000,000
Common shares, shares issued	45,276,999	40,385,309
Treasury shares (in shares)	11,315,889	10,785,007