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Stockholders' Equity - RSU Activities (Details) - Restricted Stock Units - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Number of Shares      
Outstanding, beginning balance 2,019    
RSUs granted 1,176    
RSUs vested and released (971)    
RSUs forfeited (191)    
Outstanding, ending balance 2,033 2,019  
Weighted Average Grant Date Fair Value Per Share      
Outstanding, beginning balance, weighted-average grant date fair value per share $ 7.65    
RSUs granted, weighted-average grant date fair value per share 18.29 $ 9.71 $ 5.10
RSUs vested and released, weighted-average grant date fair value per share 8.85    
RSUs forfeited, weighted-average grant date fair value per share 11.42    
Outstanding, ending balance, weighted-average grant date fair value per share 12.81 7.65  
Weighted average grant date fair value per share of RSUs granted $ 18.29 $ 9.71 $ 5.10
Total fair value of RSUs vested during the period $ 21,161 $ 10,706 $ 2,907