Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Statement of Financial Position [Abstract]
Fixed maturities, amortized cost	$ 18,583.1	$ 16,287.1	$ 13,708.0
Nonredeemable preferred stocks, cost	700.6	734.2	726.6
Common equities, cost	1,485.5	1,437.5	1,576.1
Short-term investments, amortized cost	4,311.5	3,572.9	5,876.2
Premiums receivable, allowance for doubtful accounts	189.3	186.8	176.8
Reinsurance recoverables, paid losses and loss adjustment expenses	76.4	83.8	67.5
Intangible Assets, Accumulated Amortization	157.7	109.5	94.0
Property and equipment, accumulated depreciation	$ 908.4	$ 845.8	$ 830.1
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)	215,900,000	217,600,000	216,700,000