Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023	Sep. 30, 2023
Common shares, par value (USD per share)	$ 1.00	$ 1.00	$ 1.00
Common shares, authorized (shares)	900,000,000.0	900,000,000.0	900,000,000.0
Common shares, issued (shares)	797,600,000	797,600,000	797,600,000
Treasury shares	211,800,000	212,300,000	212,600,000
Fixed maturities, amortized cost	$ 74,595.2	$ 62,441.9	$ 60,490.6
Short-term investments, amortized cost	756.5	1,789.9	1,795.2
Nonredeemable preferred stocks, cost	759.7	977.1	976.9
Common equities, cost	733.5	706.0	662.6
Premium receivable, allowance for credit loss	388.0	369.1	361.2
Property and equipment, accumulated depreciation	$ 1,589.7	$ 1,655.1	$ 1,596.9
Serial Preferred Stock
Preferred stock, shares authorized (shares)	20,000,000.0	20,000,000.0	20,000,000.0
Series B Preferred Stock | Serial Preferred Stock
Preferred stock, shares authorized (shares)	0	500,000	500,000
Preferred stock, no par value		$ 0	$ 0
Preferred stock, liquidation preference per share (USD per share)		$ 1,000	$ 1,000
Preferred stock, shares issued (shares)	0	500,000	500,000
Preferred stock, shares outstanding (shares)	0	500,000	500,000