Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Statement Of Financial Position [Abstract]
Available for sale securities, amortized cost	$ 9,552,397	$ 8,887,734
Held to maturity securities, allowance for credit losses	2,959	2,610
Held to Maturity, Fair value	$ 1,012,129	$ 1,029,444
Common stock, par value	$ 1.00	$ 1.00
Common stock, shares authorized	80,000,000	80,000,000
Common stock, shares issued	55,056,730	55,056,730
Common stock, shares outstanding	48,302,634	48,006,386
Treasury stock, shares	6,754,096	7,050,344