CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) (Unaudited) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Allowance for doubtful accounts	$ 1,561	$ 2,023
Common stock, par value	$ 1.50	$ 1.50
Common stock, shares authorized	200,000,000	200,000,000
Common stock, shares issued	24,934,981	24,934,981
Common stock, shares outstanding	24,934,981	24,934,981
Treasury stock, shares	4,601,494	4,518,099