Consolidated Balance Sheets (Parenthetical) (USD $) In Millions, except Per Share data, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
Fixed maturities, amortized cost	$ 13,644.9	$ 11,373.9	$ 11,864.0
Nonredeemable preferred stocks, cost	442.1	404.0	430.1
Common equities, cost	1,422.3	1,370.3	1,101.5
Short-term investments, amortized cost	1,146.1	1,990.0	2,394.1
Premiums receivable, allowance for doubtful accounts	133.7	138.6	133.5
Reinsurance recoverables, paid losses and loss adjustment expenses	36.4	38.9	32.5
Property and equipment, accumulated depreciation	$ 665.8	$ 625.0	$ 603.7
Common Shares, par value (USD per share)	$ 1.00	$ 1.00	$ 1.00
Common Shares, authorized (shares)	900.0	900.0	900.0
Common Shares, issued (shares)	797.6	797.7	797.7
Common Shares, treasury shares (shares)	197.6	193.1	193.0