Consolidated Balance Sheet (Unaudited) (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Available-for-sale securities, amortized cost	$ 37,741.5	$ 41,155.7
Held to maturity, fair value	20,692.8	17,797.4
Unearned income	$ 9.9	$ 9.8
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)	207,661,247	208,289,178
Treasury stock (in shares)	37,510,277	36,882,346
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