Annual report pursuant to Section 13 and 15(d)

CONSOLIDATED BALANCE SHEETS (Parenthetical)

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CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Statement Of Financial Position [Abstract]        
Common stock, par value $ 0.001 $ 0.001    
Common stock, shares authorized 300,000,000 300,000,000    
Common stock, shares issued 121,000,903 120,657,254    
Common stock, shares outstanding 116,475,033 116,210,252    
Treasury stock, shares 4,525,870 4,447,002 4,183,504 4,057,135