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Share-Based Payment Awards - Stock Option Activity Under Company's Equity Incentive Plan (Detail) - Equity Incentive Plans and Inducement Awards [Member]
6 Months Ended
Jun. 30, 2020
$ / shares
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Number of Options Outstanding, Beginning balance | shares 10,909,800
Number of Options, Granted | shares 3,613,897
Number of Options, Forfeited | shares (437,482)
Number of Options, Expired | shares (413,865)
Number of Options Outstanding, Ending balance | shares 13,672,350
Number of Options, Exercisable at June 30, 2020 | shares 5,689,656
Weighted Average Exercise Price Outstanding, Beginning balance | $ / shares $ 2.52
Weighted Average Exercise Price, Granted | $ / shares 1.30
Weighted Average Exercise Price, Forfeited | $ / shares 1.84
Weighted Average Exercise Price, Expired | $ / shares 3.70
Weighted Average Exercise Price Outstanding, Ending balance | $ / shares 2.18
Weighted Average Exercise Price, Exercisable at June 30, 2020 | $ / shares $ 2.74
Weighted Average Remaining Contractual Life, Outstanding at June 30, 2020 7 years 11 months 4 days
Weighted Average Remaining Contractual Life, Exercisable at June 30, 2020 6 years 5 months 19 days