Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Statement Of Financial Position [Abstract]
Allowances	$ 10,116	$ 9,755
Common stock, par value	$ 1	$ 1
Common stock, shares authorized	60,000,000	60,000,000
Common stock, shares issued	26,070,787	25,894,782
Treasury stock, shares	3,561,509	3,470,084