Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016
Statement of Financial Position [Abstract]
Fixed maturities, amortized cost	$ 18,311.7	$ 16,287.1	$ 13,409.6
Nonredeemable preferred stocks, cost	666.0	734.2	745.5
Common equities, cost	1,472.3	1,437.5	1,558.6
Short-term investments, amortized cost	3,729.7	3,572.9	5,166.4
Premiums receivable, allowance for doubtful accounts	175.8	186.8	159.1
Reinsurance recoverables, paid losses and loss adjustment expenses	79.2	83.8	73.8
Property and equipment, accumulated depreciation	894.7	845.8	824.0
Finite-Lived Intangible Assets, Accumulated Amortization	$ 140.5	$ 109.5	$ 78.5
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	797,500,000	797,500,000
Common Shares, treasury shares (shares)	216,500,000	217,600,000	215,600,000