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	200,000,000	200,000,000
Common stock, shares issued	108,300,000	107,800,000
Treasury stock, shares	18,700,000	18,200,000