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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]
$ / shares in Units, $ in Thousands
3 Months Ended
Mar. 31, 2020
USD ($)
$ / 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 (31,668)
Number of Options, Expired | shares (287,367)
Number of Options Outstanding, Ending balance | shares 14,204,662
Number of Options, Exercisable at December 31, 2019 | shares 4,913,150
Weighted Average Exercise Price Outstanding, Beginning balance | $ / shares $ 2.52
Weighted Average Exercise Price, Granted | $ / shares 1.30
Weighted Average Exercise Price, Forfeited | $ / shares 1.57
Weighted Average Exercise Price, Expired | $ / shares 3.63
Weighted Average Exercise Price Outstanding, Ending balance | $ / shares 2.19
Weighted Average Exercise Price, Exercisable at December 31, 2019 | $ / shares $ 2.95
Weighted Average Remaining Contractual Life, Outstanding at March 31, 2020 8 years 1 month 17 days
Weighted Average Remaining Contractual Life, Exercisable at March 31, 2020 6 years 1 month 13 days
Aggregate Intrinsic Value, Outstanding at March 31, 2020 | $ $ 515