Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Fixed maturities, amortized cost	$ 24,061.8	$ 20,209.9	$ 18,311.7
Nonredeemable preferred stocks, cost	677.0	698.6	666.0
Common equities, cost	1,314.0	1,499.0	1,472.3
Short-term investments, amortized cost	3,231.2	2,869.4	3,729.7
Premiums receivable, allowance for doubtful accounts	210.5	210.9	175.8
Reinsurance recoverables, paid losses and loss adjustment expenses	117.6	103.3	79.2
Intangible Assets, Accumulated Amortization	211.7	175.7	140.5
Property and equipment, accumulated depreciation	$ 984.7	$ 940.6	$ 894.7
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
Common Shares, issued (shares)	797,500,000	797,500,000	797,500,000
Common Shares, treasury shares (shares)	215,000,000	215,800,000	216,500,000
Serial Preferred Stock [Member]
Preferred Stock, Shares Authorized (shares)	20,000,000	20,000,000	20,000,000
Series B Preferred Stock [Member]
Preferred Stock, No Par Value	$ 0		$ 0
Preferred Stock, Liquidation Preference Per Share (USD per share)		$ 0	0
Series B Preferred Stock [Member] | Serial Preferred Stock [Member]
Preferred Stock, No Par Value	$ 0	$ 0	$ 0
Preferred Stock, Shares Authorized (shares)	500,000.0	0	0
Preferred Stock, Shares Issued (shares)	500,000.0	0	0
Preferred Stock, Shares Outstanding (shares)	500,000.0	0	0
Preferred Stock, Liquidation Preference Per Share (USD per share)	$ 1,000