Consolidated Balance Sheets (Parenthetical) - USD ($) shares in Millions, $ in Millions	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014
Fixed maturities, amortized cost	$ 15,520.6	$ 13,374.2	$ 12,265.4
Nonredeemable preferred stocks, cost	632.4	590.4	497.1
Common equities, cost	1,351.7	1,289.2	1,265.2
Short-term investments, amortized cost	1,669.3	2,149.0	3,118.7
Premiums receivable, allowance for doubtful accounts	140.6	152.2	133.4
Reinsurance recoverables, paid losses and loss adjustment expenses	46.2	46.0	36.2
Finite-Lived Intangible Assets, Accumulated Amortization	16.3	0.6	0.6
Property and equipment, accumulated depreciation	$ 770.3	$ 731.0	$ 706.3
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	797.6	797.6
Common Shares, treasury shares (shares)	211.7	209.8	206.1