Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015	[1]
Statement Of Financial Position [Abstract]
Available-for-sale cost	$ 3,008.8	$ 2,971.0
Equity securities cost	345.6	349.7
Other investments cost	518.7	506.9
Short-term investments, Cost	$ 289.2	$ 211.2
Common shares, par value	$ 1.00	$ 1.00
Common shares, shares authorized	500,000,000	500,000,000
Common shares, shares issued	39,980,160	37,104,294
Treasury shares, shares	9,996,840	9,181,644

[1] Derived from audited consolidated financial statements.