Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Held to Maturity, Fair value	$ 1,091,640	$ 1,207,447
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	49,847,139	49,894,990
Treasury stock, shares	5,209,591	5,161,740