Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
Statement Of Financial Position [Abstract]
Common stock, shares par value	$ 0.06	$ 0.06
Common stock, shares authorized	150,000,000	150,000,000
Common stock, shares issued	91,317,000	85,316,000
Common stock, shares outstanding	91,317,000	85,316,000
Treasury stock, shares	3,359,000	3,264,000