Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Investment securities:
Securities held to maturity, fair value (in dollars)	$ 1,607,184	$ 957,183
Shareholders' equity:
Common stock, par value (in dollars per share)	$ 2.50	$ 2.50
Common stock, shares authorized	160,000,000	160,000,000
Common stock, shares issued	69,918,037	70,973,477
Common stock, shares outstanding	69,918,037	70,973,477