Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016	[1]
Statement Of Financial Position [Abstract]
Available-for-sale cost	$ 3,248.6	$ 2,938.8
Equity securities cost	321.5	335.2
Other investments cost	530.0	531.6
Short-term investments, Cost	$ 365.9	$ 405.5
Common shares, par value	$ 1.00	$ 1.00
Common shares, shares authorized	500,000,000	500,000,000
Common shares, shares issued	40,262,441	40,042,330
Treasury shares, shares	10,075,255	10,028,755

[1] Derived from audited consolidated financial statements.