Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Fixed maturities, amortized cost	$ 44,556.0	$ 35,589.1	$ 35,082.8
Short-term investments, amortized cost	1,088.7	5,218.5	4,667.8
Nonredeemable preferred stocks, cost	1,479.9	1,358.7	1,302.7
Common equities, cost	1,238.4	1,187.3	1,130.0
Allowance for credit loss, beginning of period	257.3	356.2	313.2
Property and equipment, accumulated depreciation	1,422.2	1,291.4	1,264.5
Intangible Assets, Accumulated Amortization	$ 369.2	$ 326.1	$ 312.0
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	797,500,000	797,500,000
Common Shares, treasury shares (shares)	212,600,000	212,300,000	211,900,000
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, No Par Value	$ 0	$ 0	$ 0
Preferred Stock, Shares Authorized (shares)	500,000	500,000	500,000
Preferred Stock, Shares Issued (shares)	500,000	500,000	500,000
Preferred Stock, Shares Outstanding (shares)	500,000	500,000	500,000
Preferred Stock, Liquidation Preference Per Share (USD per share)	$ 1,000	$ 1,000	$ 1,000