Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Statement Of Financial Position [Abstract]
Available for sale securities, amortized cost	$ 8,887,734	$ 7,323,980
Held to maturity securities, allowance for credit losses	2,610	0
Held to Maturity, Fair value	$ 1,029,444	$ 1,082,345
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	48,006,386	49,097,606
Treasury stock, shares	7,050,344	5,959,124