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Stockholders' Equity - Summary of Stock Option Activity Under Equity Incentive Plan (Details)
 12 Months Ended
Dec. 31, 2017
$ / shares
shares
Number of Shares  
Beginning Balance (shares) | shares 6,806,183
Granted (shares) | shares 1,788,121
Exercised (shares) | shares (452,973)
Cancelled (shares) | shares (446,461)
Ending Balance (shares) | shares 7,694,870
Weighted-Average Exercise Price  
Beginning Balance ($ per share) | $ / shares $ 6.81
Granted ($ per share) | $ / shares 6.62
Exercised ($ per share) | $ / shares 5.18
Cancelled ($ per share) | $ / shares 8.49
Ending Balance ($ per share) | $ / shares $ 6.77