Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015
Fixed maturities, amortized cost	$ 13,708.0	$ 15,347.9	$ 15,539.6
Nonredeemable preferred stocks, cost	726.6	674.2	652.4
Common equities, cost	1,576.1	1,494.3	1,487.0
Short-term investments, amortized cost	5,876.2	2,172.0	2,132.0
Premiums receivable, allowance for doubtful accounts	176.8	164.8	153.3
Reinsurance recoverables, paid losses and loss adjustment expenses	67.5	46.1	46.1
Finite-Lived Intangible Assets, Accumulated Amortization	94.0	47.4	31.8
Property and equipment, accumulated depreciation	$ 830.1	$ 778.3	$ 758.8
Common Shares, par value (USD per share)	$ 1.00	$ 1.00	$ 1.00
Common Shares, authorized (shares)	900,000,000	900,000,000.0	900,000,000.0
Common Shares, issued (shares)	797,500,000	797,600,000	797,600,000
Common Shares, treasury shares (shares)	216,700,000	214,000,000	213,000,000