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CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Statement Of Financial Position [Abstract]    
Allowances for doubtful accounts $ 4,706 $ 4,701
Common stock par value (in dollars per share) $ 0.001 $ 0.001
Common stock, shares authorized 500,000,000 500,000,000
Common stock, shares issued 93,119,988 90,952,185
Convertible preferred stock, par value (in dollars per share) $ 0.001 $ 0.001
Convertible preferred stock, shares authorized 50,000,000 50,000,000
Convertible preferred stock, shares outstanding 0 0
Treasury stock, shares 24,883,000 24,867,000