Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Allowance for doubtful accounts	$ 1,661	$ 1,761
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	120,000,000	120,000,000
Common stock, shares issued	54,361,980	54,361,980
Treasury stock, shares	10,403,296	10,748,127