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Equity Plans and Stock-Based Compensation - Option Activity under Stock-Based Compensation Plans (Detail)
$ / shares in Units, shares in Thousands, $ in Thousands
 3 Months Ended
Mar. 31, 2020
USD ($)
$ / shares
shares
Shares Underlying Outstanding Options  
Beginning balance | shares 8,006
Options granted | shares 1,335
Ending balance | shares 9,260
Vested and expected to vest at March 31, 2020 | shares 8,901
Exercisable at March 31, 2020 | shares 4,826
Weighted-Average Exercise Price Per Share  
Beginning balance | $ / shares $ 13.86
Options granted | $ / shares 5.34
Ending balance | $ / shares 12.64
Vested and expected to vest at March 31, 2020 | $ / shares 12.91
Exercisable at March 31, 2020 | $ / shares $ 18.06
Weighted-Average Remaining Contractual Term (years)  
Balance at March 31, 2020 4 years 6 months 10 days
Vested and expected to vest at March 31, 2020 4 years 5 months 12 days
Exercisable at March 31, 2020 2 years 11 months 8 days
Aggregate Intrinsic Value  
Balance at March 31, 2020 | $ $ 6
Vested and expected to vest at March 31, 2020 | $ $ 5
Unvested  
Shares Underlying Outstanding Options  
Options cancelled | shares (41)
Weighted-Average Exercise Price Per Share  
Options cancelled | $ / shares $ 6.77
Vested  
Shares Underlying Outstanding Options  
Options cancelled | shares (40)
Weighted-Average Exercise Price Per Share  
Options cancelled | $ / shares $ 18.93