Consolidated Balance Sheets (Unaudited) (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Available-for-sale securities, amortized cost	$ 25,941.9	$ 23,659.0
Held to maturity, fair value	21,641.4	24,473.0
Unearned income	$ 6.6	$ 5.9
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)	204,591,724	205,126,224
Treasury stock, (in shares)	40,579,800	40,045,300
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