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Share-Based Payment Awards - Stock Option Activity Under Company's Equity Incentive Plan (Detail) - Equity Incentive Plans [Member]
$ / shares in Units, $ in Thousands
9 Months Ended
Sep. 30, 2019
USD ($)
$ / shares
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Number of Options Outstanding, Beginning balance | shares 8,139,377
Number of Options, Granted | shares 4,474,096
Number of Options, Exercised | shares (166,760)
Number of Options, Forfeited | shares (1,361,109)
Number of Options, Expired | shares (473,346)
Number of Options Outstanding, Ending balance | shares 10,612,258
Number of Options, Exercisable at September 30, 2019 | shares 4,957,932
Weighted Average Exercise Price Outstanding, Beginning balance | $ / shares $ 2.83
Weighted Average Exercise Price, Granted | $ / shares 2.36
Weighted Average Exercise Price, Exercised | $ / shares 1.84
Weighted Average Exercise Price, Forfeited | $ / shares 2.49
Weighted Average Exercise Price, Expired | $ / shares 3.77
Weighted Average Exercise Price Outstanding, Ending balance | $ / shares 2.65
Weighted Average Exercise Price, Exercisable at September 30, 2019 | $ / shares $ 3.07
Weighted Average Remaining Contractual Life, Outstanding at September 30, 2019 7 years 8 months 8 days
Weighted Average Remaining Contractual Life, Exercisable at September 30, 2019 6 years 1 month 9 days
Aggregate Intrinsic Value, Outstanding at September 30, 2019 | $ $ 289
Aggregate Intrinsic Value, Exercisable at September 30, 2019 | $ $ 41