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Stockholders' Equity - June 2021 Equity Offering (Details) - USD ($) $ / shares in Units, $ in Thousands | 1 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended
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Jun. 30, 2021 | Jan. 31, 2020 | Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2021 | Sep. 30, 2020 | Dec. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Amount paid for clearing fees | $ 19 | $ 231 |  |  |  |  |
Payments of Stock Issuance Costs |  |  | $ (1) | $ 0 | $ (470) | $ (474) | $ (510)
Proceeds from sale of common stock and warrants | $ 4,500 | $ 2,300 |  |  |  |  |
Institutional investors |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Sale of common stock (in shares) | 990,100 | 688,360 |  |  |  |  |
Sale of common stock (USD per share) | $ 5.05 | $ 3.37 |  |  |  |  |
Placement agent |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Amount paid for placement agent commissions | $ 400 | $ 193 |  |  |  |  |
Amount paid related to expenses for registered direct offering and concurrent private placement | $ 51 | $ 50 |  |  |  |  |

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