CONSOLIDATED BALANCE SHEETS (UNAUDITED) (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Available-for-sale [Abstract]
Securities available for sale, amortized cost	$ 2,650,237	$ 2,437,513
Held-to-maturity Securities, Other Disclosure Items [Abstract]
Securities held to maturity, fair value	$ 98,035	$ 118,198
Shareholders' equity:
Common stock, par value (in dollars per share)	$ 1.25	$ 1.25
Common stock, shares authorized (in shares)	80,000,000	80,000,000
Common stock, shares issued (in shares)	37,960,148	37,934,819
Treasury stock (in shares)	5,687,226	4,983,645