Consolidated Balance Sheet (Unaudited) (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022
Available-for-sale securities, amortized cost	$ 25,464.4	$ 27,760.0
Held to maturity, fair value	22,823.1	22,879.3
Unearned income	$ 8.3	$ 9.0
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)	208,342,348	208,428,309
Treasury stock (in shares)	36,829,176	36,743,215
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