Consolidated Balance Sheets (Parenthetical) - USD ($)	Mar. 31, 2021	Dec. 31, 2020
Statement Of Financial Position [Abstract]
Interest bearing deposits, banks	$ 550,008,000	$ 721,086,000
Held-to-maturity securities, fair values	$ 839,872,000	$ 768,183,000
Preferred stock, no par value
Preferred stock, shares authorized	1,000,000	1,000,000
Preferred stock, shares issued	150,000	150,000
Preferred stock, shares outstanding	150,000	150,000
6.75% non-cumulative perpetual preferred stock, Series A, liquidation preference	$ 150,000,000	$ 150,000,000
Common stock, par value	$ 2.0833	$ 2.0833
Common stock, shares authorized	100,000,000	100,000,000
Common stock, shares issued	68,081,306	68,081,306
Common stock, shares outstanding	67,282,134	67,254,706
Treasury stock, shares	799,172	826,600