Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Statement Of Financial Position [Abstract]
Available for sale securities, amortized cost	$ 8,116,473	$ 7,323,980
Held to maturity securities, allowance for credit losses	3,305	0
Held to Maturity, Fair value	$ 1,153,833	$ 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	48,021,707	49,097,606
Treasury stock, shares	7,035,023	5,959,124