Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Statement Of Financial Position [Abstract]
Interest bearing deposits, banks	$ 410,734	$ 51,891
Held-to-maturity securities, fair values	$ 841,120	$ 874,523
Preferred stock, no par value
Preferred stock, shares authorized	1,000,000	1,000,000
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 2.0833	$ 2.0833
Common stock, shares authorized	100,000,000	100,000,000
Common stock, shares issued	68,078,116	68,078,116
Common stock, shares outstanding	67,058,155	67,824,428
Treasury stock, shares	1,019,961	253,688