Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	150,000,000	150,000,000
Common stock, shares issued	12,521,000	12,257,000
Common stock, shares outstanding	11,101,000	10,844,000
Purchase of treasury shares	1,420,000	1,413,000