CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Statement Of Financial Position [Abstract]
Allowance for doubtful accounts	$ 2,023	$ 1,342
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	4,518,099	4,613,520