Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale, Amortized Cost	$ 9,897.6	$ 8,537.3
Available-for-sale Equity Securities, Amortized Cost Basis	3,269.7	3,089.1
Debt Securities, Held-to-maturity, Fair Value	$ 0.0	$ 1,058.2
Preferred Stock, Par or Stated Value Per Share	$ 0.01	$ 0.01
Preferred Stock, Shares Authorized	75,000,000	75,000,000
Preferred Stock, Shares Outstanding	0	0
Common Stock, Par or Stated Value Per Share	$ 1.00	$ 1.00
Common Stock, Shares Authorized	500,000,000	500,000,000
Common Stock, Shares, Issued	304,122,180	303,652,553
Common Class B [Member]
Common Stock, Par or Stated Value Per Share	$ 1.00	$ 1.00
Common Stock, Shares Authorized	100,000,000	100,000,000
Common Stock, Shares, Issued	0	0