Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Statement Of Financial Position [Abstract]
Held to Maturity, Fair value	$ 1,122,660	$ 1,070,532
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	49,058,246	49,117,222
Treasury stock, shares	5,998,484	5,939,508