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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
12 Months Ended
Dec. 31, 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 5,510,432
Number of Options, Exercised | shares (223,426)
Number of Options, Forfeited | shares (1,483,063)
Number of Options, Expired | shares (1,033,520)
Number of Options Outstanding, Ending balance | shares 10,909,800
Number of Options, Exercisable at December 31, 2019 | shares 4,718,213
Weighted Average Exercise Price Outstanding, Beginning balance | $ / shares $ 2.83
Weighted Average Exercise Price, Granted | $ / shares 2.19
Weighted Average Exercise Price, Exercised | $ / shares 1.85
Weighted Average Exercise Price, Forfeited | $ / shares 2.49
Weighted Average Exercise Price, Expired | $ / shares 3.39
Weighted Average Exercise Price Outstanding, Ending balance | $ / shares 2.52
Weighted Average Exercise Price, Exercisable at December 31, 2019 | $ / shares $ 3.05
Weighted Average Remaining Contractual Life, Outstanding at December 31, 2019 7 years 8 months 23 days
Weighted Average Remaining Contractual Life, Outstanding at December 31, 2019 5 years 11 months 23 days
Aggregate Intrinsic Value, Outstanding at December 31, 2019 | $ $ 539
Aggregate Intrinsic Value, Exercisable at December 31, 2019 | $ $ 46