Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
Statement Of Financial Position [Abstract]
Common stock, shares authorized	20,000,000	20,000,000
Common stock, stated value	$ 5.00	$ 5.00
Common stock, shares issued	4,728,267	4,728,267
Common stock, shares outstanding	4,420,136	4,420,255
Treasury stock, shares	308,131	308,012