Consolidated Balance Sheet (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Available-for-sale securities, amortized cost	$ 41,659.2	$ 38,722.2
Unearned income	14.6	14.1
Held to maturity, fair value	$ 16,746.5	$ 12,249.3
Preferred stock, authorized (in shares)	10,000,000	10,000,000
Common stock, par value (in dollars per share)	$ 1.6667	$ 1.6667
Common stock, authorized (in shares)	560,000,000	560,000,000
Common stock, outstanding (in shares)	208,121,233	209,709,046
Treasury stock (in shares)	37,050,291	35,462,478
Series C Preferred Stock
Preferred stock, outstanding (in shares)	0	16,000
Series D Preferred Stock
Preferred stock, outstanding (in shares)	5,000	5,000
Series E Preferred Stock
Preferred stock, outstanding (in shares)	16,000	16,000