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Stock-Based Compensation - Stock options & RSU activity (Details) - USD ($) $ / shares in Units, $ in Thousands | 1 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Jan. 31, 2021 | Sep. 30, 2021 | Sep. 30, 2020 | Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019
Options |  |  |  |  |  |
Outstanding balance at the beginning of the period (in shares) | 6,393,853 | 6,393,853 | 4,024,566 | 6,393,853 | 4,024,566 |
Granted (in shares) |  |  |  | 2,482,650 | 2,662,500 |
Exercised (in shares) |  |  |  | (2,292) | (32,778) |
Forfeited (in shares) |  |  |  | (348,593) | (260,435) |
Outstanding balance at the end of the period (in shares) |  |  |  | 8,525,618 | 6,393,853 | 4,024,566
Options exercisable at the end of the period (in shares) |  |  |  | 5,421,641 |  |
Options vested and expected to vest at end of period (in shares) |  |  |  | 8,525,618 |  |
Weighted Average Exercise Price Per Share |  |  |  |  |  |
Outstanding balance at the beginning of the period (in dollars per share) | $ 5.29 | $ 5.29 | $ 7.48 | $ 5.29 | $ 7.48 |
Granted (in dollars per share) |  | $ 1.37 | $ 1.33 | 2.19 | 2.10 |
Exercised (in dollars per share) |  |  |  | 1.55 | 1.53 |
Forfeited (in dollars per share) |  |  |  | 3.17 | 6.95 |
Outstanding balance at the end of the period (in dollars per share) |  |  |  | 4.48 | $ 5.29 | $ 7.48
Exercisable at the end of the period (in dollars per share) |  |  |  | 5.83 |  |
Options vested and expected to vest at end of the period (in dollars per share) |  |  |  | $ 4.48 |  |
Weighted Average Remaining Life in Years |  |  |  |  |  |
Outstanding balance |  |  |  | 7 years 6 months 29 days | 7 years 11 months 15 days | 7 years 11 months 23 days
Options exercisable at end of period |  |  |  | 6 years 10 months 20 days |  |
Options vested and expected to vest at end of the period |  |  |  | 7 years 6 months 29 days |  |
Aggregate Intrinsic Value |  |  |  |  |  |
Outstanding balance |  |  |  | $ 7,673 | $ 1,961 | $ 2
Options exercisable at the end of the period |  |  |  | 4,074 |  |
Options vested and expected to vest at end of the period |  |  |  | $ 7,673 |  |
Assumptions used to determine grant-date fair value |  |  |  |  |  |
Expected Volatility |  |  |  | 67.64% | 66.94% |
Weighted average risk-free interest rate |  |  |  | 0.82% | 0.66% |
Expected dividend yield |  |  |  | 0.00% | 0.00% |
Expected term (in years) |  |  |  | 6 years 9 months 25 days | 6 years 10 months 6 days |
Expiration period |  |  |  | 10 years |  |
Compensation cost related to the non-vested awards not yet recognized |  |  |  | $ 3,921 |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract] |  |  |  |  |  |
Stock-based compensation expense |  |  |  | 3,686 | $ 2,776 |
Research and development |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract] |  |  |  |  |  |
Stock-based compensation expense |  |  |  | 1,641 | 1,172 |
General and administrative |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract] |  |  |  |  |  |
Stock-based compensation expense |  |  |  | $ 2,045 | $ 1,604 |
Weighted average |  |  |  |  |  |
Assumptions used to determine grant-date fair value |  |  |  |  |  |
Weighted average period for recognition of compensation cost |  |  |  | 1 year 11 months 4 days |  |
Minimum |  |  |  |  |  |
Assumptions used to determine grant-date fair value |  |  |  |  |  |
Vesting period |  |  |  | 3 years |  |
Maximum |  |  |  |  |  |
Assumptions used to determine grant-date fair value |  |  |  |  |  |
Vesting period |  |  |  | 4 years |  |
Restricted stock units to purchase common stock |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] |  |  |  |  |  |
Settled in cash | (92,500) |  |  |  |  |
Restricted stock units to purchase common stock | Executive Officer [Member] |  |  |  |  |  |
Assumptions used to determine grant-date fair value |  |  |  |  |  |
Vesting period |  |  |  | 3 years |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] |  |  |  |  |  |
Awarded |  |  |  | 275,000 | 660,606 |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract] |  |  |  |  |  |
Stock-based compensation expense |  |  |  | $ 530 | $ 248 |
Restricted stock units to purchase common stock | 2016 Plan |  |  |  |  |  |
Assumptions used to determine grant-date fair value |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value |  |  |  | $ 1.76 | $ 1.52 | $ 1.76
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] |  |  |  |  |  |
Outstanding at the beginning | 753,106 | 753,106 | 90,500 | 753,106 | 90,500 |
Awarded |  |  |  | 275,000 | 753,106 |
Settled in cash |  |  |  | (92,500) | (90,500) |
Outstanding at the end |  |  |  | 935,606 | 753,106 | 90,500
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract] |  |  |  |  |  |
Outstanding at beginning( in dollars per share) | $ 1.52 | $ 1.52 | $ 1.76 | $ 1.52 | $ 1.76 |
Awarded (in dollars per share) |  |  |  | 2.57 | 1.49 |
Settled in cash(in dollars per share) |  |  |  | 1.97 | 1.76 |
Outstanding at end(in dollars per share) |  |  |  | $ 1.76 | $ 1.52 | $ 1.76

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- Definition The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition Per share or unit weighted-average fair value of nonvested award under share-based payment arrangement. Excludes share and unit options. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2)(iii) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2)(ii) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition Amount by which current fair value of underlying stock exceeds exercise price of fully vested and expected to vest options outstanding. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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